Fix wayland support on systems without X11.

Bug: https://bugzilla.libsdl.org/show_bug.cgi?id=5270

--- a/configure	Tue Aug 25 04:05:36 2020 +0200
+++ a/configure	Tue Aug 25 06:40:03 2020 -0700
@@ -22306,6 +22306,7 @@
           #define LINUX
           #define EGL_API_FB
           #define MESA_EGL_NO_X11_HEADERS
+          #define EGL_NO_X11
           #include <EGL/egl.h>
           #include <EGL/eglext.h>

--- a/configure.ac	Tue Aug 25 04:05:36 2020 +0200
+++ a/configure.ac	Tue Aug 25 06:40:03 2020 -0700
@@ -2373,6 +2373,7 @@
           #define LINUX
           #define EGL_API_FB
           #define MESA_EGL_NO_X11_HEADERS
+          #define EGL_NO_X11
           #include <EGL/egl.h>
           #include <EGL/eglext.h>
         ],[