xfree86: Fix meson overwriting suid wrapper with the wrapped executable

Meson gets confused when there are two targets of the same name within the same directory, so we use a different intermediate name.

This is a problem with the Xorg SUID wrapper which has the same filename as the real Xorg executable and is configured in the same meson.build file. This MR works around this by using a different filename in build stage and renaming only when installing.

Fixes #539 (closed).