m4 should check for gnugetopt, gnuregex already present on the system

[Mikhail Teterin]

FreeBSD, as well as -- I'm sure -- all Linux distributions already have
both the GNU getopt and the GNU regex functionality available -- either
as part of (g)libc or as additional libraries.

Using those will help cut the m4-executable's size (30% on my
FreeBSD/amd64), as well as avoid the bugs already fixed by the later
revisions of these libraries.

m4's configure should check for these and avoid compiling m4's own
regex.o and getopt*.o

Attached is the patch, I'm trying to include into FreeBSD port of m4 --
it should help illustrate, what I'm talking about.

Thanks! Yours,

        -mi

patch-no-redundancy
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