Preliminary version of new regex matcher for gawk now available

Hi All.

I've been working with Mike Haertel (the original author of GNU grep)
for a number of months now.  He is writing a new regexp matcher for
use in gawk (and other places, as people desire).

The matcher is avalable on Github: https://github.com/mikehaertel/minrx.
I have created a branch in the gawk repo that uses it: feature/minrx.

MinRX is currently written in C++20. Mike will eventually rewrite it
in C for portability.  For the moment, you'll need to use gcc / g++
to build the branch.  I haven't tried to mess with clang / clang++.

The test suite passes completely.

The new matcher is the default, so that it will be exercised.  The old
matchers are still available. To use them, set GAWK_GNU_MATCHERS in
the environment.   I will NOT make a formal release with MinRX as long
as MinRX is still in C++.

For now, the only way to access the code is via Git:

	git clone https://git.savannah.gnu.org/r/gawk.git
	cd gawk
	git checkout feature/minrx
	./bootstrap.sh && ./configure && make -j && make check

If you use gawk, please try this branch out.

Questions, comments, and *bug reports* are welcome.

Thanks,

Arnold