When a project has a bundled copy of our macro included in tarball
source releases, typically in a m4/ or aclocal/ directory, aclocal
uses the serial number to determine whether the system copy in
/usr/share/aclocal is newer or older than the bundled copy. Without
a serial number, the bundled copy will always be used and will not
be overwritten, even if it is outdated.

Signed-off-by: Simon McVittie <smcv@debian.org>