build: avoid Xlib.h via EGL headers

This should make EGL headers not pull in Xlib headers when no specific platform
define is in effect.

Use both the old Mesa-specific hack and the new official define EGL_NO_X11 to
have this work on both old and new EGL headers.

Fixes: https://gitlab.freedesktop.org/wayland/weston/issues/350

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>

meson.build

@@ -99,6 +99,8 @@ env_modmap = ''
 
 config_h.set('_GNU_SOURCE', '1')
 config_h.set('_ALL_SOURCE', '1')
+config_h.set('EGL_NO_X11', '1')
+config_h.set('MESA_EGL_NO_X11_HEADERS', '1')
 
 config_h.set_quoted('PACKAGE_STRING', 'weston @0@'.format(version_weston))
 config_h.set_quoted('PACKAGE_VERSION', version_weston)