Add support for local non-root install

Daniel van Vugt requested to merge vanvugt/gnome-shell:fix-install into master

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 /usr/...

So modify keysdir to only use the official path from pkg-config when it is physically possible to honour it. Otherwise use the locally configured --prefix, which is the same as the existing behaviour when gnome-keybindings.pc does not exist.

Fixes: #399 (closed)

Alternative solutions: !144 (closed), !145 (closed)

Edited by Daniel van Vugt

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