ROOT does not like an empty rootloogff.C file
cd to SRCDIR before make version; fixes out-of-source.
gitinfo.sh requires that the current working area is the tag.
Source tar prefix is now versioned (as everyone else does it).
Do not build as part of make version. Do the git commit.

make version now only updates the sources without building anything.
The git commit is also automated to have the same log style thoughout.
Be helpful with what comes next by mentioning the release howto.
Pass build dir to find root.exe.
Handle more failures; pick up objdir as $1.
Improve make version.