wish: conditionally build man page (tor.1) and html doc using independent configure options

Currently it is practice in Gentoo Linux to install at least the the man page. However that creation is coupled to the html files too (right ?) and therefore forces the Tor package to pull in more - otherwise completely unneeded - dependent packages.

De-coupling tor.1 from the html docs would at least allow us in the Gentoo Linux universe to get rid of packages like libxslt and friends at a dedicated Tor relay.