This requires the setting of the --with-gcc-toolchain option in ./configure,
in addition to the --with-cc and --with-c++ options, which are then passed
to the clang/LLVM build procedure.


git-svn-id: http://root.cern.ch/svn/root/trunk@48841 27541ba8-7e3a-0410-8455-c3a389f83636