When detecting that we need to rebuild LLVM (current revision number of the llvm directory is 
different from the one when build), delete the file llvmrev.txt and recreate it *only* after
a successfull build so that we insure re-running the llvm build until it is completed.
(This fixes problems induce by running another target after svn update or interrupting the
build with CTRL+C, etc...)


git-svn-id: https://root.cern.ch/svn/root/trunk@48719 27541ba8-7e3a-0410-8455-c3a389f83636
correct name of the pch.


git-svn-id: https://root.cern.ch/svn/root/trunk@48720 27541ba8-7e3a-0410-8455-c3a389f83636
always run onepcm as part of the "postbin" target which is a dependency
of "all".


git-svn-id: https://root.cern.ch/svn/root/trunk@48721 27541ba8-7e3a-0410-8455-c3a389f83636