libwildmidi: update to 0.4.6.

Bug-fix and maintenance update after the previous version 0.4.5.
All users of wildmidi-0.4.x are encouraged to upgrade.

What's new in 0.4.6:

    A lot of player cleanup and refactoring, thanks to initial work
    by Azamat H. Hackimov, with addition of several safeguards and
    minor fixes.
    Ability to choose which audio output backends to include in the
    build system: see the cmake script for the relevant WANT_???
    options. Player's --help command line switch lists the available
    backends. Thanks to initial work by Azamat H. Hackimov.
    New native audio output backends for player: coreaudio for macOS,
    sndio for OpenBSD, netbsd (sunaudio) for NetBSD.
    Improved pkg-config file generation in cmake script (bug #236).
    Workaround a link failure on AmigaOS4 with newer SDKs (bug #241).
    Other minor source clean-ups.
    CMake project clean-ups. Cmake v3.4 or newer is now required.

(wiz)