Add configuration for the optional package pythia8.

To select pythia8, in your configure file add something like
 --enable-pythia8 \
 --with-pythia8-incdir=$HOME/pythia8100/include \
 --with-pythia8-libdir=$HOME/pythia8100/lib \


git-svn-id: http://root.cern.ch/svn/root/trunk@22069 27541ba8-7e3a-0410-8455-c3a389f83636