meson: *-user: only descend into *-user when configured

meson: *-user: only descend into *-user when configured

To increase flexibility, only descend into *-user when that is
configured. This allows *-user to selectively include directories based
on the host OS which may not exist on all hosts. Adopt Paolo's
suggestion of checking the configuration in the directories that know
about the configuration.

Message-Id: <20210926220103.1721355[email protected]>
Message-Id: <20210926220103.1721355[email protected]>
Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Signed-off-by: Warner Losh <[email protected]>
Acked-by: Paolo Bonzini <[email protected]>
Reviewed-by: Kyle Evans <[email protected]>