native Cocoa backend will be build by default. To build X11 backend, install
XQuartz and configure with ./configure --disable-cocoa. On OSX < 10.8 X11
backend is still default and Cocoa backend can be build using the 
--enable-cocoa configure flag.


git-svn-id: http://root.cern.ch/svn/root/trunk@45327 27541ba8-7e3a-0410-8455-c3a389f83636