cmake: Use common installation directory for cmake support files.

[rhabacker]

At least on openSUSE, the usual installation location for cmake support files is ${CMAKE_INSTALL_LIBDIR}/cmake/

$ find /usr/lib64 -name '.cmake' | grep /cmake/ | wc -l 834 $ find /usr/lib64 -name '.cmake' | grep -v /cmake/ | wc -l 8

which is also used by glslang with these changes.