configure.ac: rephrase 'GLX w/o X11' error message

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
diff --git a/configure.ac b/configure.ac
index 06883a9..1da67f5 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1716,7 +1716,7 @@
 
 if test "x$enable_glx" != xno; then
     if ! echo "$platforms" | grep -q 'x11'; then
-        AC_MSG_ERROR([Building without the x11 platform as GLX is enabled, is not supported])
+        AC_MSG_ERROR([Building GLX without the x11 platform is not supported])
     fi
 fi