Make libvpx optional.
By default, this feature is turned off until we figure out a neat way to download pre-built mangled libvpx at configure time. However, the CI pipelines will continue to build libvpx.
By default, this feature is turned off until we figure out a neat way to download pre-built mangled libvpx at configure time. However, the CI pipelines will continue to build libvpx.