check for sys/sysctl.h availability

Newer Linux ports are dropping sysctl support.  If you try to even
include the header, you'll get build failures:
/usr/include/bits/sysctl.h:19:3: error: #error "sysctl system call is unsupported"

Check for the header before we try to use it.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>