This is more elegant as a meson feature option because feature options
propagate in a useful way to `dependency(..., required: ...)`.
Without this, the option was ignored. Wiring it up naively meant that
disabling it still searched for libbsd as well, so a feature option
is the best fit.
Also, mark it as a dependency of `arc4random`.
Signed-off-by: Sam James <sam@gentoo.org>
