gobjectIntrospection: add giDiscoverSelf preFixupPhase hook

Some derivations need to discover their own typelibs. giDiscoverSelf
runs as a preFixupPhase and populates GI_TYPELIB_PATH with an entry
pointing to $prefix/lib/girepository-1.0, if it exists.

pkgs/development/libraries/gobject-introspection/setup-hook.sh

@@ -1,5 +1,4 @@
 make_gobject_introspection_find_gir_files() {
-
     # required for .typelib files, eg mypaint git version
     if [ -d "$1/lib/girepository-1.0" ]; then
       addToSearchPath GI_TYPELIB_PATH $1/lib/girepository-1.0
@@ -13,9 +12,16 @@ make_gobject_introspection_find_gir_files() {
 
 envHooks+=(make_gobject_introspection_find_gir_files)
 
+giDiscoverSelf() {
+    if [ -d "$prefix/lib/girepository-1.0" ]; then
+      addToSearchPath GI_TYPELIB_PATH $prefix/lib/girepository-1.0
+    fi
+}
+
+preFixupHooks+=(giDiscoverSelf)
+
 _multioutMoveGlibGir() {
   moveToOutput share/gir-1.0 "${!outputDev}"
 }
 
 preFixupHooks+=(_multioutMoveGlibGir)
-