Require GNU M4 1.4.8 or later.

Require GNU M4 1.4.8 or later.

The former minimum version was 1.4.6.  1.4.6 and 1.4.7 do not track
the original location of text fed to ‘m4wrap’, which breaks autom4te’s
ability to trace macros invoked from _AC_FINALIZE.

Right now, the only _user_ visible effect of this is that autoconf
running on M4 1.4.6 or 1.4.7 will emit an internal error, instead of
the intended warning message, when it processes a configure.ac that
neglects to invoke AC_INIT or AC_OUTPUT.  Perhaps more importantly,
it causes a bunch of scary-sounding failures in our own testsuite,
which deliberately doesn’t use AC_OUTPUT sometimes.

M4 1.4.6 and 1.4.7 also have bugs in location tracking of macro
invocations spread over multiple lines, which, guess what, causes
even more testsuite failures.

1.4.8 came out in 2006.  As a practical matter, this change to our
requirements means that people using macOS *to run autoconf* (not just
to run generated configure scripts) cannot use the system-provided m4
anymore.  {Free,Net,Open}BSD already don’t ship GNU M4 as their system
m4, so users of those OSes should not be affected.