gl: Allow build even if no window system is enabled

Submitted by Olivier Blin

Link to original bug (#776578)

Description

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).