xfree86: Create a symlink X -> Xorg when installing using meson

This brings the behavior closer than what we currently have with
autotools-based build system.

Meson does not currently have native support for symlinks.
See https://github.com/mesonbuild/meson/issues/1602.

Signed-off-by: Povilas Kanapickas <povilas@radix.lt>
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