- add support for 'clang' compiler, which was not the case before.  
- add support for 'cocoa'
- Added rpath link options "@loader_path/../lib" for executables and
  "@rpath" for libraries for MacOSX platform


git-svn-id: http://root.cern.ch/svn/root/trunk@43836 27541ba8-7e3a-0410-8455-c3a389f83636