CMake: Use GNUInstallDirs for pkgconfig too
CMake: Use GNUInstallDirs for pkgconfig too

Fixes i.e. multilib setups on linux

Follow-up to 5681911965ef

libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat,libexpat/libexpat