m4sh: Require shell to support $(...) command substitution.

m4sh: Require shell to support $(...) command substitution.

As of the 2020-11-07 update, config.sub and config.guess
unconditionally use $(...) command substitution; see
<https://lists.gnu.org/archive/html/config-patches/2020-11/msg00011.html>.

Therefore, add this to the set of required shell features, searched
for by _AS_DETECT_BETTER_SHELL.  On a system where /bin/sh doesn’t
support $(...), $CONFIG_SHELL will be set to one that does (and the
primary configure script will be re-executed using that shell).
AC_CANONICAL_* use $CONFIG_SHELL to execute config.guess/sub, so they
will keep working.  This also means that configure scripts and
third-party macros that use $(...) will quietly start working
correctly on such ancient systems.

The test code is simple, but sufficient to weed out Solaris 10’s
/bin/sh, which doesn’t support $(...) but *does* support shell
functions.

I’m not going to touch any of the existing uses of `...` command
substitution in Autoconf proper for now, but it might make sense to
bulk upgrade them early in the 2.71 release cycle; if nothing else,
it would remove a major obstacle to running shellcheck over our
scripts.

* lib/m4sugar/m4sh.m4 (_AS_MODERN_CMDSUBST_WORKS): New macro.
  (AS_INIT, AS_SHELL_SANITIZE): Call _AS_DETECT_REQUIRED for
  _AS_MODERN_CMDSUBST_WORKS.
* NEWS: Mention the requirement for $(...).