Here's the problem: I have to write out a configuration file that, after building expat, points to where the libraries are installed - and do so portably, so that on an OS that uses $LD_LIBRARY_PATH it'll write:

LD_LIBRARY_PATH=/path/to/expat:$LD_LIBRARY_PATH; export LD_LIBRARY_PAT
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...and on OS's that use $DYLD_LIBRARY_PATH the same thing, and on windows a similar manipulation of %PATH%.