Create some toplevel cmake defines to disable certain features

[jackjansen]

If we add toplevel defines to disable features that are unimplementable on certain platforms that should greatly help the port to Android (and possibly to other platforms).

A few come to mind:

• CWIPC_WITHOUT_PYTHON disables all Python interfaces.
• CWIPC_WITHOUT_APPS disables all applications.
• CWIPC_WITHOUT_GLFW disables cwipc_window(), no need for glfw and OpenGL.

For the camera support we have to see whether to disable them all in one fell swoop, or maybe simply have them not be built if the corresponding API package isn't available.

[jackjansen]

This was implemented as part of the Android port.