Omit htonll when already #defined

OS X 10.10 Yosemite defines it as a macro.

Signed-off-by: R. Andrew Ohana <andrew.ohana@gmail.com>
[rlb@defaultvalue.org: adjust comments; adjust commit message]
Reviewed-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>