Build: explicitly allow skipping Publican building docs

There are cases one wants to avoid Publican being involved in the build
process at all (for instance, if a local copy is too old to behave
gracefully with what build process requires).

We avoid introducing a new configure option by (sort of) overloading
--with-brand one.

See also rhbz#1326350 (instructing configure using --with-brand=
would then help).