Make sure the llvmrev.txt is created in the right directory (and thus actually used) and quiet down the output


git-svn-id: https://root.cern.ch/svn/root/trunk@48722 27541ba8-7e3a-0410-8455-c3a389f83636
Improve error detection and reporting.  Do not build the standalone pch as we do not use and ... not everybody as clang++ in their path


git-svn-id: https://root.cern.ch/svn/root/trunk@48723 27541ba8-7e3a-0410-8455-c3a389f83636
Add missing dependency


git-svn-id: https://root.cern.ch/svn/root/trunk@48724 27541ba8-7e3a-0410-8455-c3a389f83636
on linux casing matters


git-svn-id: https://root.cern.ch/svn/root/trunk@48725 27541ba8-7e3a-0410-8455-c3a389f83636