gl: Allow build even if no window system is enabled
Submitted by Olivier Blin
The gst-gl plugins and libraries should be allowed to build even if no window system is enabled.
On platforms that do not provide a standard GL window system, this is useful for applications using gst-gl with their own sink, like WebKit with GSTREAMER_GL enabled.
When no window system is supported, glimagesink will use a dummy window (see gst_gl_window_new).