Adds a support for compiling flutter engine when
gdk does not have X11 backend. In such a configuration
the generated gdkconfig.h header file looks like the following:

 /* gdkconfig.h
  *
  * This is a generated file.  Please modify `configure.ac'
  */

 #ifndef __GDKCONFIG_H__
 #define __GDKCONFIG_H__

 #if !defined (__GDK_H_INSIDE__) && !defined (GDK_COMPILATION)
 #error "Only <gdk/gdk.h> can be included directly."
 #endif

 #include <glib.h>

 G_BEGIN_DECLS

 #define GDK_WINDOWING_WAYLAND

 G_END_DECLS

 #endif  /* __GDKCONFIG_H__ */

Additionally headers like <gdk/gdkx.h> are not available at all.

Above configuration can be found on the most of the embedded systems.

This patch enables compilation of X11 specific code only when gdk
defines GDK_WINDOWING_X11.
Signed-off-by: NDamian Wrobel <dwrobel@ertelnet.rybnik.pl>