meson: use system dependencies instead of subproject

[marcusmueller]

Since PMT(f) is rather central, let's keep the default source the subproject, but add an external_pmt meson option.

This mirrors what https://github.com/gnuradio/pmt/pull/46 did to pmt (where it only affected CLI11 selection).

The motivation here is that we can now have a pmtf distro package, and let the newsched distro package build-depend on it.