avoid regenerating defsincdate (use shipped file)

avoid regenerating defsincdate (use shipped file)

upstream ships doc/defsincdate in its tarballs.  but doc/Makefile.am
tries to rewrite doc/defsincdate if it notices that any of the files
have been modified more recently, and it does so assuming that we're
running from a git repo.

However, we'd rather ship the documents cleanly without regenerating
defsincdate -- we don't have a git repo available (debian builds from
upstream tarballs) and any changes to the texinfo files (e.g. from
debian/patches/) might result in different dates on the files than we
expect after they're applied by dpkg or quilt or whatever, which makes
the datestamp unreproducible.

Gbp-Pq: Topic debian-packaging
Gbp-Pq: Name 0003-avoid-regenerating-defsincdate-use-shipped-file.patch