Add Configure_ac.pm to set of files to be synced from automake.

Add Configure_ac.pm to set of files to be synced from automake.

This file seems to have been accidentally left off of the list of .pm
files that should be kept the same between automake and autom4te.
It’s already almost identical in both codebases.

It would make sense to sync ChannelDefs.pm too, but the copies of that
file in the two codebases have diverged; they would need to be
reconciled by hand first.

* build-aux/fetch.sh (automake_files): Add lib/Automake/Configure_ac.pm.