Add GLIB_PREFIX variable
Add GLIB_PREFIX variable

The gtk-doc-fixref script needs to know the location of the
libraries documentation; in order to get it, we can ask the
pkg-config file of the library for its prefix.

robtaylor/json-glib-gvariant,Distrotech/json-glib,brauliobo/json-glib,oerdnj/json-glib,GNOME/json-glib,oerdnj/json-glib,oerdnj/json-glib,brauliobo/json-glib,GNOME/json-glib,robtaylor/json-glib-gvariant,Distrotech/json-glib,ebassi/json-glib,oerdnj/json-glib,ebassi/json-glib,brauliobo/json-glib,frida/json-glib,ebassi/json-glib,Distrotech/json-glib,frida/json-glib