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.

configure

@@ -4548,7 +4548,7 @@ int main(int argc, char *argv[]) {
 EOF
   _gl=no
   for _ld_tmp in "" -lGL "-lGL -lXdamage" "-lGL $_ld_pthread" ; do
-    if cc_check $_ld_tmp $_ld_lm ; then
+    if test "$_cocoa" != yes && cc_check $_ld_tmp $_ld_lm ; then
       _gl=yes
       _gl_x11=yes
       libs_mplayer="$libs_mplayer $_ld_tmp $_ld_dl"