Apparently, some *BSD variants (and maybe some other OSes) does not define
SIGPWR signal.  So since commit 760fc6debc, busybox fails to build on
such platforms.  Fix this.
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>