xvkbd: explicitly set LIBDIR and CONFDIR
xvkbd: explicitly set LIBDIR and CONFDIR

In preparation to fix long-standing `imake` bug #135337 we need
to set `LIBDIR` and `CONFDIR` away from incorrectly embedded
`xorg-cf-files`. Before #135337 the package worked by chance.

Issue: https://github.com/NixOS/nixpkgs/issues/135337

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