--- a/configure	Thu Sep 20 21:17:49 2012 -0700
+++ b/configure	Thu Sep 20 22:01:51 2012 -0700
@@ -18638,7 +18638,10 @@
         case "$host" in
             *-*-darwin*)
                 # This isn't necessary for X11, but fixes GLX detection
-                if test "x$x_includes" = xNONE && test "x$x_libraries" = xNONE; then
+                if test "x$x_includes" = xNONE && \
+                   test "x$x_libraries" = xNONE && \
+                   test -d /usr/X11R6/include && \
+                   test -d /usr/X11R6/lib; then
                     x_includes="/usr/X11R6/include"
                     x_libraries="/usr/X11R6/lib"
                 fi

--- a/configure.in	Thu Sep 20 21:17:49 2012 -0700
+++ b/configure.in	Thu Sep 20 22:01:51 2012 -0700
@@ -1012,7 +1012,10 @@
         case "$host" in
             *-*-darwin*)
                 # This isn't necessary for X11, but fixes GLX detection
-                if test "x$x_includes" = xNONE && test "x$x_libraries" = xNONE; then
+                if test "x$x_includes" = xNONE && \
+                   test "x$x_libraries" = xNONE && \
+                   test -d /usr/X11R6/include && \
+                   test -d /usr/X11R6/lib; then
                     x_includes="/usr/X11R6/include"
                     x_libraries="/usr/X11R6/lib"
                 fi