meson.build: Add support for local non-root install
To configure a build for local installation you can use:
meson --prefix $HOME/foo ...
which mostly works. Only
keysdir is hardcoded to ignore it and
would fail as non-root upon running
ninja install because non-root
can't write to the usual
keysdir to only use the official path from
it is physically possible to honour it. Otherwise use the locally
--prefix, which is the same as the existing behaviour when
gnome-keybindings.pc does not exist.