Draft: Allow building only the daemon

For source-based distributions it'd be very nice to be able to build the daemon against a previously installed libpulse install (from a -Ddaemon=false build). This merge request is a way to achieve this in a dirty way for me in Gentoo Linux, but of course there'd be much more to this to do more properly upstream (separate translation files/domains, bash completion script, etc). It's a straight rebase on top of master, whereas I built this originally against v15.0 tag and that's with what it's been tested with, not master yet. It is used in Gentoo split packages (separate libpulse and pulseaudio-daemon packages, as opposed to pulseaudio with a daemon USE flag), but they are "unkeyworded" for other reasons so far. The first commit ("Relax sanity checks") is applied to libpulse package instead and it can probably be considered separately in a new MR?

@igor.v.kovalenko asked me to file this for discussion, so here it is.

Perhaps even a repository-level libpulse/pulseaudio split could be useful these days.