This change fixes a few things in one go:
* cmake's FindOpenGL cannot be used reliably to detect EGL. So use a
custom module for that.
* Added a custom module for GLESv2 detection, as cmake's FindOpenGL
does not support that.
* Map CONFIG += opengl to a WrapOpenGL target, which links against
either GLESv2 or libGL - just like mkspecs/features/*/opengl.prf
* cmake's FindOpenGL remains in use solely to detect the availability
of desktop gl.
Change-Id: I9315e5ad1fd88e1b7dc7e920053e98fb51fea7fc
Reviewed-by: Volker Krause <volker.krause@kdab.com>
