add --enable-generate-man-pages configure switch (default: enabled)

This forces generation of man pages, even if cached ones exists. This
"fixes" a typical release tarball nit. While it is hackish, the
benefit is clear given the history of failed tarball releases since
we changed the cached man page handling. It was just too easy to get
that wrong.
Merge branch 'v7-stable' into v8-stable