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 <jacknagel@gmail.com>
