Mingw: generated FAudio.pc file has incorrect/incomplete paths.

[aur-r]

When using MinGW, the generated "FAudio.pc" contains incorrect or incomplete paths.
libdir=${prefix}
includedir=${prefix}
While using include(GNUInstallDirs with MinGW may have been an issue when "FAudio.pc" was introduced into the code, that is no longer the case in my testing.

faudio-CMAKE_INSTALL-dirs.patch.txt

Maybe someone can look into it.

[flibitijibibo]

Can merge a fix for this whenever someone gets to it - I don't actually think we have any users of this right now; FNA doesn't install the fnalibs and Wine statically links it into their XAudio2 binaries, so these lines can probably be changed without much consequence.