Use EGL_NO_X11 to disable X11 headers

MESA_EGL_NO_X11_HEADERS has been deprecated, and libglvnd only supports
EGL_NO_X11.

Fixes: #214

src/dispatch_common.h

@@ -47,9 +47,12 @@
 #endif
 #if PLATFORM_HAS_EGL
 # if !ENABLE_X11
-/* Mesa uses this symbol to avoid including X11 headers when including
+/* Disable including X11 headers if the X11 support was disabled at
- * EGL.h; since X11 was explicitly disabled at configuration time, we
+ * configuration time
- * should do the same
+ */
+#  define EGL_NO_X11 1
+/* Older versions of Mesa use this symbol to achieve the same result
+ * as EGL_NO_X11
  */
 #  define MESA_EGL_NO_X11_HEADERS 1
 # endif