build ROOT.pch only with headers part of ./configure --gminimal.


git-svn-id: https://root.cern.ch/svn/root/trunk@48822 27541ba8-7e3a-0410-8455-c3a389f83636
fix for out-of-source build.


git-svn-id: https://root.cern.ch/svn/root/trunk@48823 27541ba8-7e3a-0410-8455-c3a389f83636