[release,bugfix] Makes PKG_CONFIG_PATH honor all (lib lib/multiarch).

Previously some of the pathes (e.g the multiarch ones) took precedence
over the others. But as with same some of the pkgconfig files are installed
below lib/<multiarch> and others (if there is not object library) are not.
With this patch all possible install locations of pkgconfig files are added
to the path if they exist.