Extend, don't replace GI_TYPELIB_PATH in gnome-shell launcher

Without this we can't support some gi typelibs being in system
directories.

src/gnome-shell.in

@@ -195,6 +195,8 @@ def start_shell(perf_output=None):
                 'NO_AT_BRIDGE'        : '1'})
 
     if running_from_source_tree:
+        if os.environ.has_key('GI_TYPELIB_PATH'):
+            typelib_dir = typelib_dir + ":" + os.environ.get('GI_TYPELIB_PATH')
         env.update({'GNOME_SHELL_DATADIR'  : data_dir,
                     'GI_TYPELIB_PATH'      : typelib_dir,
                     'GSETTINGS_SCHEMA_DIR' : data_dir })