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: Kyle J. McKay <mackyle@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>