configure: disable X11 opengl backend if Cocoa is enabled
The recommended way to get function pointers to the functions in the
OpenGL library is through dlopen/dlsym/dlclose. This causes problems
in the Cocoa OpenGL backend when -lGL (X11's OpenGL headers) is linked
to the binary together with -framework OpenGL.
The linked OpenGL symbols are always from -lGL, causing all the
function pointers to point to null when getFunctions is called against
a Cocoa OpenGL context.
For this reason change the configure autodetection code to disable
the vo_gl X11 backend when cocoa is active.