determine the GL platform to use at runtime

- Make GstGLWindow subclassablerather than specified at compile time.
- Add GstGLWindowX11 for x11 windows and two subclasses, GstGLWindowX11GLX
  and GstGLWindwX11EGL for GLX and EGL repectively. (win32 and cocoa
  ports to come)
- Also cleanup GL library detection in configure.ac
fixup configure.ac for a mingw build
remove get_platform and get_window_handle vfuncs

- rename choose_visual() to choose_format() for later incorporating
  win32 changes and new GstGLPlatform for WGL, EGL, GLX, etc.
add GST_GL_PLATFORM_ANY for choosing any platform
win32 port of determining GL platform at runtime

- add GstGLWindowWin32 and GstGLWindowWin32WGL along the same lines as the
  X11 variants
WindowX11EGL: remove get_platform call
cocoa port of determining gl platform at runtime
fix cocoa videooverlay example
add configure support for GNUstep Cocoa libraries
remove GNUstep makefiles
fix displaying of configure --help
window_cocoa: fix segfault with gst-launch-1.0 -e ... and sending SIGINT (Ctrl-C)

GNUstep has a signal handler for SIGINT which runs in the main thread.
However that thread has never been 'registered' with GNUstep so
the autorelease magic of objective-c causes a segfault from accessing
a null NSThread object somewhere deep in GNUstep.
try GLX first when using ANY | OPENGL | OPENGL3
preliminary support for Win32 EGL
fixup GNUstep check for cross compilation
move window-specific code into seperate subfolders
fix 'set but not used' compiler warnings
add wayland-egl window backend
fix typo for win32 debug variables
fix configure checks for wayland, egl and glx enablement