glib-2.28.x: blind commit that hopefully fixes a lot of build errors on Darwin

It appears that users of glibc expect to be able to link libintl via pkgconfig.
On Linux, this always works because that library comes with glibc, but on other
platforms gettext is required to fulfill the dependency.

I don't have access to Darwin, so I cannot actually test this change. If any
runs into problems because of this modification, please feel free to revert it!

svn path=/nixpkgs/trunk/; revision=31723