In order to further simplify "unprivileged" builds, add "udev" boolean,
which make installing Udev rules optional.

I.e. without requiring any ugly hacks (mktemp -d), an privileged build
capable of running PulseAudio and JACK applications, can be configured
trivially:

meson --prefix=~/.local build
meson configure build -Dsystemd-user-unit-dir=$HOME/.config/systemd/user
meson configure build -Dpipewire-alsa=false
meson configure build -Dudev=false

Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@iki.fi>