Extensions to M4sh

[Alex Ameen]

Hey I recently implemented a pretty sizeable set of M4sh extensions that
make it much more suitable for writing general purpose shell scripts.

I've been using them for my own scripts for a while now, and intend to use
them in the future to refactorparts of `ltmain.in` to be generated in
before releasing source tarballs. With this in mind, users wouldn't require
any of these macros to build `libtool`, but it would allow maintainers ( me
) and contributors to significantly reduce boilerplate.

While I could certainly check these into `libtool`, I think `autoconf`
might be a more appropriate home for them; but I wanted to check-in before
dropping a giant patch on y'all. Given that `libtool` could potentially be
blocked by pending patches in `autoconf` in the future, I understand if
that's an issue for y'all. In practice I could build `libtool` releases
with unreleased or patched macros vendored in `libtool` or `gnulib`; but
that's not really something I could realistically expect contributors to
work with.

In any case, I do think these would be a great addition to M4sh, and make
Autom4te more useful outside of the `autotools` domain. Whoever is most
actively working on M4sh would be an incredibly useful contact for me, so
if "that's you" let me know.