diff --git a/configure.ac b/configure.ac
index 3237858..1d0eb27 100644
--- a/configure.ac
+++ b/configure.ac
@@ -157,6 +157,8 @@ if test x$enable_x11 = xno; then
         AC_MSG_ERROR([GLX support is explicitly enabled, but X11 was disabled])
     fi
     build_glx=no
+else
+    AC_DEFINE([ENABLE_X11], [1], [Whether X11 support is enabled])
 fi
 
 AM_CONDITIONAL(BUILD_EGL, test x$build_egl = xyes)