I am well aware of how pkg-config and custom prefixes and install dirs work.
That doesn't change the fact that I said 'to mirror the meson.build for VapourSynth':
https://github.com/HolyWu/L-SMASH-Wo...eson.build#L65
And that this means it goes into the same plugin directory VapourSynth itself creates and uses for the plugins that come with the upstream source. Something which the meson.build for AviSynth is not doing at the current time.