Whatever you did, it messed up the command line that g++ is invoked with:

g++ Sybase.def -o blib\arch\auto\DBD\Sybase\Sybase.xs.dll -mdll -s -L"
+C:\STRAWB~1\perl\lib\CORE" -L"C:\STRAWB~1\c\lib"-L"c:\SAP\OCS-16_0\dl
+l\" Sybase.o ...
[download]

Note how these escaped bckslashes and missing spaces in your addition are unlike the other -L stanzas.

A first step would be to make these identical to the other -L stanzas and add proper whitespace:

g++ Sybase.def -o blib\arch\auto\DBD\Sybase\Sybase.xs.dll -mdll -s -L"
+C:\STRAWB~1\perl\lib\CORE" -L"C:\STRAWB~1\c\lib" -L "c:\SAP\OCS-16_0\
+dll" Sybase.o
[download]

Also, there should be a way to pass the proper parameters to Makefile.PL from the command line instead of patching Makefile.PL.

Thanks. I fixed this. It still doesn't solve the problem (please see below).

Most of these libraries exist (and the few others are most likely not important indeed). I changed Makefile.PL to -lsybct (..), but got the same result. I set the LIBRARY_PATH then, which reduced the warnings I got from Makefile.PL to

Warning (mostly harmless): No library found for -lsybtcl
Warning (mostly harmless): No library found for -lsybcomn
Warning (mostly harmless): No library found for -lsybintl
[download]

This is much better, because these libraries are not installed by the Sybase driver, but gmake still results in

c:\Strawberry\cpan\build\DBD-Sybase-1.17-0>gmake
Skip blib\lib\DBD\dbd-sybase.pod (unchanged)
Skip blib\lib\DBD\Sybase.pm (unchanged)
Running Mkbootstrap for Sybase ()
"C:\Strawberry\perl\bin\perl.exe" -MExtUtils::Command -e chmod -- 644 
+"Sybase.bs"
"C:\Strawberry\perl\bin\perl.exe" -MExtUtils::Command::MM -e cp_nonemp
+ty -- Sybase.bs blib\arch\auto\DBD\Sybase\Sybase.bs 644
gcc -c  -IC:\SAP/OCS-16_0/include -DNO_BLK=1 -IC:/Strawberry/perl/vend
+or/lib/auto/DBI -DWIN32 -DWIN64 -D__USE_MINGW_ANSI_STDIO -DPERL_TEXTM
+ODE_SCRIPTS -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO 
+-fwrapv -fno-strict-aliasing -mms-bitfields -s -O2   -DVERSION=\"1.17
+\" -DXS_VERSION=\"1.17\"  "-IC:\STRAWB~1\perl\lib\CORE"   Sybase.c
In file included from C:\STRAWB~1\perl\lib\CORE/perl.h:5603,
                 from C:/Strawberry/perl/vendor/lib/auto/DBI/DBIXS.h:2
+3,
                 from Sybase.h:16,
                 from Sybase.xs:14:
Sybase.xs: In function 'XS_DBD__Sybase__db_ping':
Sybase.xs:84:32: warning: implicit declaration of function 'syb_ping';
+ did you mean 'syb_init'? [-Wimplicit-function-declaration]
     ST(0) = sv_2mortal(newSViv(syb_ping(dbh, imp_dbh)));
                                ^~~~~~~~
C:\STRAWB~1\perl\lib\CORE/embed.h:557:46: note: in definition of macro
+ 'sv_2mortal'
 #define sv_2mortal(a)  Perl_sv_2mortal(aTHX_ a)
                                              ^
Sybase.xs:84:24: note: in expansion of macro 'newSViv'
     ST(0) = sv_2mortal(newSViv(syb_ping(dbh, imp_dbh)));
                        ^~~~~~~
Sybase.xs: In function 'XS_DBD__Sybase__st_cancel':
Sybase.xs:96:13: warning: implicit declaration of function 'syb_st_can
+cel'; did you mean 'syb_st_finish'? [-Wimplicit-function-declaration]
     ST(0) = syb_st_cancel(sth, imp_sth) ? &PL_sv_yes : &PL_sv_no;
             ^~~~~~~~~~~~~
             syb_st_finish
gcc -c  -IC:\SAP/OCS-16_0/include -DNO_BLK=1 -IC:/Strawberry/perl/vend
+or/lib/auto/DBI -DWIN32 -DWIN64 -D__USE_MINGW_ANSI_STDIO -DPERL_TEXTM
+ODE_SCRIPTS -DPERL_IMPLICIT_CONTEXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO 
+-fwrapv -fno-strict-aliasing -mms-bitfields -s -O2   -DVERSION=\"1.17
+\" -DXS_VERSION=\"1.17\"  "-IC:\STRAWB~1\perl\lib\CORE"   dbdimp.c
dbdimp.c: In function 'syb_st_execute':
dbdimp.c:4076:39: warning: passing argument 2 of 'ct_results' from inc
+ompatible pointer type [-Wincompatible-pointer-types]
       while (ct_results(imp_sth->cmd, &restype) == CS_SUCCEED && rest
+ype != CS_CMD_DONE) {
                                       ^~~~~~~~
In file included from Sybase.h:19,
                 from dbdimp.c:10:
C:\SAP/OCS-16_0/include/ctpublic.h:381:10: note: expected 'CS_INT *' {
+aka 'long int *'} but argument is of type 'int *'
  CS_INT *result_type
  ~~~~~~~~^~~~~~~~~~~
dbdimp.c:4101:75: warning: passing argument 5 of 'ct_bind' from incomp
+atible pointer type [-Wincompatible-pointer-types]
             if( ct_bind(imp_sth->cmd, i+1, &phs->datafmt, SvPVX(phs->
+sv), &params[i].len, 0) != CS_SUCCEED )
                                                                      
+     ^~~~~~~~~~~~~~
In file included from Sybase.h:19,
                 from dbdimp.c:10:
C:\SAP/OCS-16_0/include/ctpublic.h:117:10: note: expected 'CS_INT *' {
+aka 'long int *'} but argument is of type 'int *'
  CS_INT *outputlen,
  ~~~~~~~~^~~~~~~~~
g++ Sybase.def -o blib\arch\auto\DBD\Sybase\Sybase.xs.dll -mdll -s -L"
+C:\STRAWB~1\perl\lib\CORE" -L"C:\STRAWB~1\c\lib" Sybase.o dbdimp.o   
+"C:\STRAWB~1\perl\lib\CORE\libperl532.a" "c:\SAP\OCS-16_0\lib\libsybc
+t.lib" "c:\SAP\OCS-16_0\lib\libsybcs.lib" "c:\SAP\OCS-16_0\lib\libsyb
+blk.lib" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libm.a" "C:\STRAWB~1\c
+\x86_64-w64-mingw32\lib\libmoldname.a" "C:\STRAWB~1\c\x86_64-w64-ming
+w32\lib\libkernel32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libuser3
+2.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libgdi32.a" "C:\STRAWB~1\c
+\x86_64-w64-mingw32\lib\libwinspool.a" "C:\STRAWB~1\c\x86_64-w64-ming
+w32\lib\libcomdlg32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libadvap
+i32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libshell32.a" "C:\STRAWB
+~1\c\x86_64-w64-mingw32\lib\libole32.a" "C:\STRAWB~1\c\x86_64-w64-min
+gw32\lib\liboleaut32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libneta
+pi32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libuuid.a" "C:\STRAWB~1
+\c\x86_64-w64-mingw32\lib\libws2_32.a" "C:\STRAWB~1\c\x86_64-w64-ming
+w32\lib\libmpr.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libwinmm.a" "
+C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libversion.a" "C:\STRAWB~1\c\x86
+_64-w64-mingw32\lib\libodbc32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\li
+b\libodbccp32.a" "C:\STRAWB~1\c\x86_64-w64-mingw32\lib\libcomctl32.a"
+ -Wl,--enable-auto-image-base
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xd51): undefin
+ed reference to `ct_options'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xe15): undefin
+ed reference to `ct_cmd_alloc'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xef4): undefin
+ed reference to `ct_config'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xf34): undefin
+ed reference to `ct_config'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xfae): undefin
+ed reference to `ct_con_alloc'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0xff1): undefin
+ed reference to `ct_con_props'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x1028): undefi
+ned reference to `ct_con_props'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x10be): undefi
+ned reference to `ct_con_props'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x114a): undefi
+ned reference to `ct_con_props'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x1188): undefi
+ned reference to `ct_con_props'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x11d4): more u
+ndefined references to `ct_con_props' follow
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x12c3): undefi
+ned reference to `cs_loc_alloc'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x130c): undefi
+ned reference to `cs_locale'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x136f): undefi
+ned reference to `cs_locale'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x13d2): undefi
+ned reference to `cs_locale'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x1427): undefi
+ned reference to `cs_dt_info'
C:/Strawberry/c/bin/../lib/gcc/x86_64-w64-mingw32/8.3.0/../../../../x8
+6_64-w64-mingw32/bin/ld.exe: dbdimp.o:dbdimp.c:(.text+0x14a9): undefi
+ned reference to `ct_config'
... (und many more lines) ...
[download]

All these ct_.. functions are located in libsybct.lib, the cs_.. functions are located in libsybcs.lib.

<UPDATE> I missed that you had effectively covered most of what I originally wrote in this post.
If libsybct.lib and libsybcs.lib are in the path specified in LIBRARY_PATH then the reason they're not being linked in is that the name specified in the "-l..." switch is incorrect.
If "-lsybct" is not working, try variations like "-llibsybct.lib", "-llibsybct", "-lsybct.lib" ... until you hit the rendition that works.

If libsybct.lib and libsybcs.lib are not in the path specified in LIBRARY_PATH, then add that location to LIBRARY_PATH:

set LIBRARY_PATH=%LIBRARY_PATH%;C:\path\to\libs;C:\path\to_other\libs
[download]

AFAICS, the only issue is that there's still one or two libs that aren't being linked in.
Perhaps there's another location that needs to be added to LIBRARY_PATH, or perhaps the wrong filename is being specified.

You can identify those libraries by doing a plain text search of all lib files for the undefined references.
It's a bit of a rough hack, but it has always worked fine for me. Once you've found the lib file that contains the string "ct_options", then you've probably found the lib file that will resolve that symbol.
Similarly, for "ct_cmd_alloc", "ct_config", etc.
My guess would be that there's one lib that resolves the undefined "ct_*" symbols and another that resolves the undefined "cs_*" symbols.
Once you've established the name(s) and location(s) of the relevant lib(s), then you should hopefully have all the info needed for you to fix the problem.

Cheers,
Rob

In order to make sure that the library does contain function references use nm -a or some windows equivalent.

But why does the Makefile runs the compiler with these 2 flags: -DWIN32 -DWIN64. They seem contradictory to me? If you are linking 32 bit libraries with 64 bit perl you are most likely have problems.

bw, bliako

libsybct.lib and libsybcs.lib are in the path specified in LIBRARY_PATH and I tried every variation of "-llibsybct.lib", "-llibsybct", "-lsybct.lib", .. None worked :(

.. None worked :(

I wonder why nothing worked.
I take it that libsybct.lib and libsybcs.lib are in the same directory as libsybtcl.lib, libsybintl.lib and libsybblk.lib.
It seems very odd that some are being located, but others are not.

Try creating copies of libsybct.lib and libsybcs.lib (in the same folder as those two files ), named libsybct.a and libsybcs.a.
Then specify the '-l' switches as -lsybct and -lsybcs
And see what happens then.

Cheers,
Rob