"?=" is not POSIX, packagers should use "make PREFIX=".
This reverts commit c225c4315161a992b9e44dd990d083ee57f7f713.

Signed-off-by: drkhsh <me@drkhsh.at>

function prototype with unspecified argument is obsolete since c99.

additionally some of these function which don't take any argument were
being called with a `const char *` arg, which is UB.

fix both these issues by declararing ALL the components to accept a
`const char *`, and name the arg "unused" if it's meant to be ignored.

- Edit compile instructions
- Reuse OpenBSD sndio implementation

Apparently `-lkvm` is needed for swap on FreeBSD

Thanks to Micheal Buch and Jason Smith
(See thread https://lists.suckless.org/dev/1907/33594.html)

Starting with OpenBSD 6.7 regular users cannot access raw audio devices
anymore, for improved security.

Instead use the sioctl_open(3) API to access and manipulate audio
controls exposed by sndiod(8). On the first call a permanent connection
is established with the running sndiod daemon, and call-back functions
are registered which are triggered when audio controls are changed
(e.g., a USB headset is attached) or when the volume is modified. On
subsequent calls we poll for changes; if there are no volume changes
this costs virtually nothing.

Joint work with Alexandre Ratchov

Based on functionality in dstat by Joerg Jung.