meson.build: Add support for local non-root install

If configured with meson --prefix ~/foo then it's likely the user will be running ninja install as non-root. This is ideal for development but will fail if dependency('gnome-keybindings', required: false) is found, because that's almost certainly in /usr/share where the user can't install to.

So make sure any custom prefix applies to the keybindings directory too, and ninja install will succeed without errors.

Fixes: #399 (closed)

Alternative solutions: !142 (closed), !145 (closed)