Split makeonepcm into configuration, generation; parametrize output file name.

makepchinput.sh determines which headers, linkdefs, flags are to be used for
ROOT's PCH. It is called by ROOT's Makefile.

makepch.sh uses that information to produce a PCH; it takes optional additional
arguments to include user headers in a user PCH replacing the default one from
ROOT.
Fire up new makepchinput.sh; copy and run makepch.sh.
Follow change in location.
Make PCH file user settable through $ROOT_PCH.
Use rootcling not rootcling_tmp. Get cppflags from $rootdir.
Move makepch.sh to etc/dictpch/ to make it accessible in installs.
Fix flag output file name.
Complete the work of Axel on user PCHs for CMake.