EXTRA_DIST, directories, tar --exclude-vcs

[Karl Berry]

Stefano and all,

It would be nice to able to list directories in EXTRA_DIST.  It is
painful and unnecessary overhead to list every file in a contrib
directory individually just to avoid including VC files.  (As explained
at
http://www.gnu.org/software/automake/manual/automake.html#Basics-of-Distribution.)

I don't propose any major surgery to make it work in every conceivable
circumstance.  All that is really necessary is to provide a way to pass
--exclude-vcs to tar.  It would only work with GNU tar, but that is ok.

What worked for me is to override the definition of am__tar, like this in
Makefile.am:
  am__tar = tar --exclude-vcs --format=ustar -chf - "$$tardir"

Except I don't really want to put this into Makefile.am.  Happily, this
also worked from the command line:
  $ env am__tar='tar --exclude-vcs --format=ustar -chf - "$tardir"' make dist

For that matter, it would be nice if am__tar provided a way to pass
arbitrary extra options.  Perhaps include something like
$(AM_TAR_USER_OPTIONS) in the definition? 

Equivalently, provide a way to override the name "tar", as in
$(AM_TAR_PROGRAM)?  That might be simplest of all.

Along those lines, it would be nice if Automake always used the -chf
style above.  In some distributions' Makefiles I've seen "chf" (didn't
look into why); that prevents adding options though, due to the vagaries
of tar command line parsing which we all know.  I have the impression
that every tar (except maybe really ancient ones, can't remember, but we
don't care) supports the -style.

Sorry for the brain dump of all these possibilities, but wdyt?

k