Add support for FreeBSD 11+.

This change adds support for FreeBSD 11, and onwards up to 19. I've
also changed the comment to correct reflect that it's FreeBSD 5+,
rather than specifically 5 to 11.

Mostly solves #26.
Make sure the test target builds dependencies.

The test target can only be run after the tests and the libstatgrab
library have been built, otherwise it reports that there's nothing
to test:

Result: NOTESTS

This change only builds the required dependencies, so it won't build
the examples or manpages.
Break the ChangeLog generation up.

This used to be split over multiple lines when it was in the Makefile,
but I concatenated it by mistake when bringing it across to here.
Add missing brackets in AC_INIT.

This was suggested by autoupdate.
Add an internal script for generating HTML docs.

This script isn't intended for distribution, but I wanted to put it
somewhere it wouldn't get lost. It can be run after the manual pages
have been built to generate HTML versions suitable for putting on the
website.