meson: add 'gpl' option to allow/disallow build of plugins with (A)GPL deps (and bump meson req to 0.59)
Require explicit opt-in to build plugins with (A)GPL dependencies, same as e.g. FFmpeg does.
Now one might say it's not like gst-plugins-ugly gets any less ugly if you don't build the GPL bits, but I think there are really entirely separate concerns here, and with the monorepo setup it's even easier now to pull in GPL-ed things automatically.
People building proprietary stuff on top of GStreamer are "terrified" of GPL-ed dependencies (their words), whilst generally being reasonably sure that they've got themselves covered on the patent front for the things that they're using or at least less bothered about it.