On BSD-compatible systems some information such as the number
of available CPUs may only be available via the sysctl function.

Add support for a HAVE_BSD_SYSCTL option complete with autoconf
support and include the sys/syctl.h header when the option is
enabled to make the sysctl function available.
Signed-off-by: NKyle J. McKay <mackyle@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>