egl: Don't force X11 support when testing for EGL.

Fixes building Wayland support on embedded systems without X11.

(TODO: maybe move the EGL test out of the X11 tests at some point, too.)

configure

@@ -21775,6 +21775,7 @@ $as_echo_n "checking for EGL support... " >&6; }
 
           #define LINUX
           #define EGL_API_FB
+          #define MESA_EGL_NO_X11_HEADERS
           #include <EGL/egl.h>
           #include <EGL/eglext.h>
 

configure.in

@@ -2340,6 +2340,7 @@ CheckOpenGLESX11()
         AC_TRY_COMPILE([
           #define LINUX
           #define EGL_API_FB
+          #define MESA_EGL_NO_X11_HEADERS
           #include <EGL/egl.h>
           #include <EGL/eglext.h>
         ],[

src/video/wayland/SDL_waylandvideo.h

@@ -33,7 +33,7 @@
 !!! FIXME:  in Ubuntu 18.04 (and other distros).
 */
 
-
+#define MESA_EGL_NO_X11_HEADERS
 #include <EGL/egl.h>
 #include "wayland-util.h"