Bypass -k in first build, so that necessary initializations are done.

Bugfix following up to my previous PR !172 (merged) which just has been merged. This bypasses the new -k a.k.a. "keep" option when doing the very first full build, so that running just make in the toplevel dir does the right thing after a fresh checkout. Previously -k would unconditionally bypass all the cleaning tar_em_up.sh does in the section starting with the comment:

# clean files that may remain stuck even after doing global make clean (if any)

This PR fixes this behaviour by just bypassing -k if no full build has been done yet (as determined by the non-existence of Gem/configure).

Tested only on Linux so far, but this shouldn't break any behavior of tar_em_up.sh before !172 (merged), as the old script didn't have the -k option at all.