Pkg-config catalogdir should be relative to datadir

In current gladeui-2.0.pc files, all the paths are absolute and resolved:

prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
datadir=/usr/share

catalogdir=/usr/share/glade/catalogs
pixmapdir=/usr/share/glade/pixmaps
moduledir=/usr/lib64/glade/modules

While this is fine for eg. libdir and datadir, paths to directories that may be populated by other packages, eg. catalogdir for external Glade catalogues, would benefit from being relative to ${datadir}. That could be:

prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
datadir=/usr/share

catalogdir=${datadir}/glade/catalogs
pixmapdir=${datadir}/glade/pixmaps
moduledir=${libdir}/glade/modules

That would allow something like the following to resolve to /usr/local/share/glade/catalogs without having to hard-code any Glade specific path:

pkg-config gladeui-2.0 --variable=catalogdir --define-variable=datadir=/usr/local/share