chore(common): Improve meson build file

Thanks @eli-schwartz for the suggestions (in #5797)!

Unfortunately we currently still have to support meson 0.45
(because that's what ships with Ubuntu 18.04), so we can't
implement everything suggested.

Also, newer meson versions always assume utf-8, but until we
can assume at least version 0.60 of meson we'll have to
explicitly specify the encoding, otherwise at least some tests
will fail.