Don't build any of the clang tools; saves considerably on link time.
Having them might still be useful for debugging, though (think "where is my STL?"), so allow them to be built with make ROOT_BUILDCLANG=1


git-svn-id: https://root.cern.ch/svn/root/trunk@48496 27541ba8-7e3a-0410-8455-c3a389f83636