always build/install manpages

[elliefm]

We have a bunch of conditionalisation in Makefile.am to try to make it so that we don't build/install man pages for features that were disabled during configure. But it's brittle, and causes headaches (e.g. #3867 (comment))

We should remove the conditionalisation, and just have all our manpages always built and installed. That'll be easier to maintain, and more robust in use.