add libplacebo to the build

[kevmitch]

Debian's version is old and crusty and a newer version of libplacebo is required for gpu-next.

I've tested building both with and without the debian package. Also tested building and linking libmpv with https://github.com/mpv-player/mpv-examples/blob/master/libmpv/simple/simple.c

[haasn]

How does this compare to #188?

[Dudemanguy]

Superseded by #188.