Migrates all MinGW 3rd-party dependencies to vcpkg except for Qt and libgig.
This enables the following for MinGW builds
- mp3/opus support
- per-note panning in Sf2 Player
- DirectSound and WASAPI audio backends in PortAudio
- LV2 plugin support
Switch back to `ENV{PKG_CONFIG}` to ensure that
FindPkgConfig doesn't overwrite `PKG_CONFIG_EXECUTABLE`.
Since CMake 3.3 supports the environment variable,
it's safe to use that.