gladeui-2.0.pc: Data paths relative to datadir

Having data directory paths, like catalogdir, being relative to datadir
allows to resolve them with an arbitrary prefix. Packages installing
Glade catalogs may use this to resolve catalogdir using their prefix.

GNOME/glade#353
3 jobs for wip/tchaik/pkg-config-libdir in 5 minutes and 51 seconds (queued for 8 seconds)
Status Job ID Name Coverage
  Test
passed #211775
flatpak

00:05:08

 
  Review
passed #211776
review

00:00:42

manual #211777
allowed to fail manual
stop_review