add support for building clang/LLVM with an alternative gcc compiler.
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: https://root.cern.ch/svn/root/trunk@48841 27541ba8-7e3a-0410-8455-c3a389f83636
fix a bug in computing the upper limit when the range of plot was different than range of variables. 
See http://root.cern.ch/phpBB3/viewtopic.php?f=15&t=16164


git-svn-id: https://root.cern.ch/svn/root/trunk@48842 27541ba8-7e3a-0410-8455-c3a389f83636