heads up RE autoconf C23 vs legacy code

Hi,

The recent autoconf upgrade broke devel/scmcvs. It is now injecting
-std=gnu23 on the command line after having regenerated the configure
script. Apparently this is by design since autoconf 2.73 but it's not
mentioned anywhere in the commit message.

There are legacy prototypes in scmcvs that don't compile with C23.
FORCE_C_STD=gnu99 seems to undo autoconf's assumptions so I guess
that's the fix we should use? Probably there are other packages that
are broken. I found macports PRs related to PHP 8.2, 8.3, 8.4 ...

Kind regards,
-Tobias