Find SC_ARG_MAX via C sysconf().

Apparently os.sysconf('SC_ARG_MAX') returns -1, even in some common
cases.

Thanks to Alexander Barton <alex@barton.de> for reporting that at
least when run under make 3.81, os.sysconf('SC_ARG_MAX') was just
returning -1.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>