If unifdef fails for any reason (like segfaulting), we should be aborting
the install steps.  So check its exit status in this unlikely scenario.
Reported-by: NDiego Elio Pettenò <flameeyes@gentoo.org>
Signed-off-by: NMike Frysinger <vapier@gentoo.org>
Signed-off-by: NMichal Marek <mmarek@suse.cz>