Make sure X paths are set before using
http://bugs.gentoo.org/79496

--- configure
+++ configure
@@ -1878,4 +1878,8 @@
 else
-  INCS="$INCS -I$x_includes"
-  LIBS="-L$x_libraries"
+  if test -n "$x_includes" ; then
+    INCS="$INCS -I$x_includes"
+  fi
+  if test -n "$x_libraries" ; then
+    LIBS="-L$x_libraries"
+  fi