autoconf's AC_PROG_GNU_M4 macro (from m4.m4) and the --gnu flag to gm4

Hi, I'm writing because I will occasionally see messages like the
following from CI systems (specifically GitHub Actions' macOS runners)
when they attempt to run autoconf:

/Applications/Xcode_15.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/gm4:
unrecognized option `--gnu'
Try `/Applications/Xcode_15.2.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/gm4
--help' for more information.
autom4te: error: gm4 failed with exit status: 1

My first attempt at fixing this was hacking in support for the `--gnu`
flag for Apple's copy of gm4, but since the apple-oss-distributions
organization has pull requests turned off, there isn't much I can
really do there to get Apple to upstream it:
https://github.com/apple-oss-distributions/gm4/compare/main...cooljeanius:apple-gm4:gnu_flag

My second attempt at fixing this was to figure out why autoconf was
passing the `--gnu` flag to m4 in the first place, and it seems that
it comes from the AC_PROG_GNU_M4 macro in m4.m4 (which seems
generally-useful, btw, and like something that autoconf should
probably ship to users). I think what is happening here is that
[g]m4's support for the `--gnu` flag gets detected at autoconf's
configure time, but then when the user overrides the M4 variable to
point to a different [g]m4, the M4_GNU variable stays set to whatever
it was first detected as, which might not match up with what the new
[g]m4 supports. I'm wondering if there's some way that autoconf could
shift around the timing of these checks so that it doesn't try passing
`--gnu` to an installation of [g]m4 that doesn't support it?

Please let me know if there's a different mailing list I should send
this request to instead.
Thanks,
Eric Gallager