added support for pre-compiled header files. Compilers currently supported:
gcc > 4.0.0
VC++ >= 7.1
A few sources had to be changed because they get now exposed to some more
includes from ROOT.

The compile time speedup can be up to 30%.


git-svn-id: http://root.cern.ch/svn/root/trunk@14290 27541ba8-7e3a-0410-8455-c3a389f83636