autoupdate should/could update the serial number of macro files it changes that have one

[Eric Gallager]

If I have an M4 macro file in my macros directory with an obsolete macro
usage in it, and I run autoupdate, it would be nice if autoupdate also
changed the serial number in it for the sake of aclocal, so that aclocal
can know that it's newer than any un-updated copies of the macro file. Just
another possible improvement I've thought of while autoupdating a large
number of macro files; I have more ideas to send later. Thanks for all your
work on autotools!

Eric Gallager