root: disable Cocoa on Darwin, use X11 front-end

ROOT expect to be compiled with LLVM/Clang on Darwin for Cocoa front-end
and uses -ObjC++ flag. The flag is not supported by GCC and produces an
error.

Signed-off-by: David Abdurachmanov <davidlt@macms07.cern.ch>
Merge pull request #584 from davidlt/fix-root-darwin

root: disable Cocoa on Darwin, use X11 front-end