Add --enable-xscreensaver to configure for USE flag support

Patch by Kevin McCarthy <signals@gentoo.org>

--- configure.ac
+++ configure.ac
@@ -316,6 +316,9 @@
 	 #endif
 	])
 
+AC_ARG_ENABLE(xscreensaver, AC_HELP_STRING([--enable-xscreensaver],
+        [compile with X Screensaver]), [have_xss=$enableval], [have_xss=no] )
+
 if test "$have_xss" = yes; then
 	# Let's figure out what library it's in...