There is no Pg.c in the distribution, only Pg.xs, which will be translated to Pg.c by the ExtUtils/xsubpp tool in your perl distribution.

Depending on a lot of aspects, line numbers in Pg.c have no meaning to people working on different versions of perl, DBI or OS.

If you build from source, you can find the line numbers after make/nmake/dmake/gmake:

$ grep ^#line Pg.c | tail -4
#line 907 "Pg.xs"
#line 2834 "Pg.c"
#line 39 "./Pg.xsi"
#line 3359 "Pg.c"
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Only the line numbers in Pg.xs will be useful to others (using the same version of DBD::Pg).

use strict;
use warnings;

use Inline C => <<'END_OF_C';

static int errno;

void foo() {
  printf("errno: %d\n", errno);
}

END_OF_C

foo();
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Microsoft (R) Program Maintenance Utility   Version 7.00.8882
Copyright (C) Microsoft Corp 1988-2000. All rights reserved.

--snip--

    cl -c  -I"C:/_32/pscrpt/inline"  -nologo -GF -W3 -MD -Zi -DNDEBUG 
+-Ox -GL -fp:precise -DWIN32 -D_CONSOLE -DNO_STRICT -DWIN64 -DCONSERVA
+TIVE -DPERL_TEXTMODE_SCRIPTS -DUSE_SITECUSTOMIZE -DPERL_IMPLICIT_CONT
+EXT -DPERL_IMPLICIT_SYS -DUSE_PERLIO -MD -Zi -DNDEBUG -Ox -GL -fp:pre
+cise    -DVERSION=\"0.00\"  -DXS_VERSION=\"0.00\"  "-IC:\_64\ap1600\l
+ib\CORE"   try_pl_3645.c
try_pl_3645.c
try_pl_3645.c(210) : error C2129: static function 'int *_errno(void)' 
+declared but not defined
        C:\_64\Platform_SDK\Include\crt\errno.h(54) : see declaration 
+of '_errno'
NMAKE : fatal error U1077: 'cl' : return code '0x2'
Stop.

A problem was encountered while attempting to compile and install your
+ Inline
C code. The command that failed was:
  C:\_64\PLATFO~1\Bin\nmake.exe > out.make 2>&1

The build directory was:
C:\_32\pscrpt\inline\_Inline\build\try_pl_3645

To debug the problem, cd to the build directory, and inspect the outpu
+t files.

 at try.pl line 12.
    ...propagated at C:/_64/ap1600/site/lib/Inline/C.pm line 772.
BEGIN failed--compilation aborted at try.pl line 12.
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There's only the one instance of "errno" in the entire DBD::Pg source, so I'm guessing there's no harm in removing that one and only instance at line 14 of Pg.h. Or am I missing something ?

https://github.com/bucardo/dbdpg/blame/master/Pg.h says it was added in 2002

oldest rt.cpan.org report is from 2004,

first mention around 2000 with Visual Studio 6.0 as prerequisite

So add something like #if _MSC_VER == 1200 ... #endif so its only added where it was historically needed