[InterNetNews/inn] aa3c12: Run support scripts with the shell found by Autoconf

[Julien ÉLIE]

  Branch: refs/heads/2.7
  Home:   https://github.com/InterNetNews/inn
  Commit: aa3c12c5ca9124005f9e57ce19873e4e47a654d1
      https://github.com/InterNetNews/inn/commit/aa3c12c5ca9124005f9e57ce19873e4e47a654d1
  Author: Julien ÉLIE <Julien-Elie at users.noreply.github.com>
  Date:   2023-01-08 (Sun, 08 Jan 2023)

  Changed paths:
    M Makefile.global.in
    M doc/pod/news.pod
    M include/Makefile
    M scripts/innshellvars.in

  Log Message:
  -----------
  Run support scripts with the shell found by Autoconf

The default shell is used when running fixconfig, fixscript, mksystem and
mkversion at build time. This shell may not be POSIX-compliant.  The one found
by Autoconf should be used instead.

The build otherwise fails with errors like:

../support/mkversion '2.8.0' '20230106 snapshot' > inn/version.h
../support/mkversion: syntax error at line 16: `date=$' unexpected

close #252