Even though xscreensaver now uses GTK+3 instead of GTK+2 since version 6.05
it still gets its directories from GTK+2 which results in an empty prefix
when GTK+2 is not installed.

Bug: https://bugs.gentoo.org/878875

Signed-off-by: Pascal Jäger <pascal.jaeger@leimstift.de>

--- a/configure
+++ b/configure
@@ -16035,7 +16035,7 @@ printf "%s\n" "$ac_cv_gtk_config_libs" >&6; }

   GTK_DATADIR=""
   if test "$have_gtk" = yes; then
-    GTK_DATADIR=`$pkg_config --variable=prefix gtk+-2.0`
+    GTK_DATADIR=`$pkg_config --variable=prefix gtk+-3.0`
     GTK_DATADIR="$GTK_DATADIR/share"
   fi

@@ -21440,6 +21440,6 @@ printf %s "checking for locale directory... " >&6; }
 if test -n "$GTK_DATADIR" ; then
   PO_DATADIR="$GTK_DATADIR"
 elif test "$have_gtk" = yes; then
-  PO_DATADIR=`$pkg_config --variable=prefix gtk+-2.0`
+  PO_DATADIR=`$pkg_config --variable=prefix gtk+-3.0`
   PO_DATADIR="$PO_DATADIR/share"
 fi