build: add datadir to pkgconfig files
Mutter wants sysprof's exact dbus interface directory during build time
On certain platforms Mutter's datadir != sysprof's datadir. This is initial work for a forthcoming PR to do
sysprof_dep.get_pkgconfig_variable('datadir')in mutter for compatibility with these platforms.
This patch is on behalf of nixpkgs where we actually have to carry a downstream patch for this exact reason.
We install everything into their own immutable prefix under /nix/store.