build: Add #serial lines to m4 files

This helps prevent outdated copies of the m4 files being used if
multiple copies are available in the search path.

See https://www.gnu.org/software/automake/manual/html_node/Serials.html



Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=765552