build: make manpages a mandatory part of the build

Currently, both man pages and gtk-doc HTML documentation are enabled by the same build option. It is common for users to want to choose whether to build HTML docs, as not everyone cares about HTML developer docs, but manpages are intended directly for end-user consumption and should always be available.

It adds no extra dependencies to the build, so enable it unconditionally.

Fixes: #1653