(EDG Binaries) Rewrote the way EDG binaries are created and found

The old scripts/bincompat-sig had a serious design flaw when it came
to finding the best EDG binary version to download: it required
knowledge of a "canonical branch" against which the EDG binaries are
generated.  It tried to find this branch by looking at the user's
remotes and trying to match against a repo URL. This tends to fail for
users that don't pull from one of the usual places.  Another lesser
issue is that the version number is not computed in an efficient way.

The new edg-generate-sig script fixes this by finding the best EDG
version number from a list of available version numbers without
requiring a "canonical branch".  See "edg-generate-sig --help" for all
the details.  The new script is also substantially smaller in terms of
LOC, and written in Bash rather than Perl.

The edg-unused-versions script can be used to generate a list of
old EDG version numbers and could be run periodically by a bot
to remove stale EDG binaries. See its "--help" switch.

I also rewrote the entire Makefile.am for src/frontend/CxxFrontend
because the old one had filled up with lots of cruft over time.

The script that uploads EDG binaries will need to be changed. It must
generate an EDG_MANIFEST file for the download site.

Conflicts:

	src/frontend/CxxFrontend/Makefile.am

Cherry-picked from b1649fdfdd4bac593b967c932f4420e725c7e48e
(Makefiles) Fixed "make clean" in the CxxFrontend directory

(cherry picked from commit a01b3650621eeddf63e7a8089430684714e84774)
Merge remote branch 'origin/master' into HEAD