m4: get rid of "# serial" lines

m4: get rid of "# serial" lines

The "#serial" lines are only considered by aclocal for the system-wide
third-party '.m4' files, not for the Automake-provided ones.  So they
serve no real purpose in the Automake '.m4' files.

In addition, now that we use git and topic branches, and that we are also
writing the Automake-NG fork, the "#serial" lines are becoming more and
more unreliable (e.g., different version of the same file in different
branches can easily end up having the same serial numbers).

So let's just nuke all the "#serial" lines.  See also automake bug#11932.

* m4/*.m4: All "# serial" lines removed.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>