win32 substitute_libpath_macro changes

For application to HEAD, following community review.

Determines the PKGLIBDIR equivalent under win32. Requires pgsql lib and bin
directories to be packaged under the same root directory (eg. /pgsql/bin and /pgsql/lib) for the win32 port, which does
not appear to be an onerous restriction.

Cheers,
Claudio

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