add Phil's skip target to the Makefile. Skip allows one to bypass all

dependency checking for a quick recompile of only the desired target, e.g.:
make skip cont/src/G__Cont.cxx
use only for testing purposes in the development process.


git-svn-id: http://root.cern.ch/svn/root/trunk@3953 27541ba8-7e3a-0410-8455-c3a389f83636