[PATCH] Add --with-diffutils option

[Justin Erenkrantz <je...@apache.org>]

Currently, it is not possible to specify which diff/diff3 combination
should be used by Subversion.  You must manually tweak your path in
order to do this.  We should be allowing --with-diffutils as an
option.  Therefore, I'd really like to see this patch make it in.

I also reworked all of the diffutils detection to be a m4 macro
rather than relying upon shell scripts.  This is a much better fit
with the rest of our autoconf code than it was previously.

Any concerns?  This works in all combinations I can test.  If no one
objects, I will commit tomorrow.  -- justin