Sort out remaining XQuartz/stdenv issues
Sort out remaining XQuartz/stdenv issues

We use PKG_CONFIG_LIBDIR to reset the default search path, overriding
whatever is baked into the pkg-config executable. This way, we can later
append XQuartz paths here while still allowing any brewed libs to take
precedence, keg-only or not.

Hopefully this will resolve any remaining issues, and let us get rid of
some per-formula hacks.

c.f. Homebrew/homebrew#14474.

Signed-off-by: Jack Nagel <43386ce32af96f5c56f2a88e458cb94cebee3751@gmail.com>

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