Upstream don't support this anymore (https://github.com/gentoo/gentoo/pull/24404#issuecomment-1061128526)
but it's a trivial patch so let's keep it going for now.
--- xscreensaver-6.03.orig/configure.ac	2022-03-04 11:23:24.415172769 -0700
+++ xscreensaver-6.03/configure.ac	2022-03-04 11:27:40.419205257 -0700
@@ -4737,7 +4737,6 @@
     CONF_STATUS=1
   elif test "$with_gl_req" = no ; then
     noteL 'The OpenGL 3D library is not being used.'
-    CONF_STATUS=1
   else
     noteL 'The OpenGL 3D library was not found.'
     CONF_STATUS=1