No need to define RANDOM_MAX

Since OS X defines RAND_MAX too, there is no need to define RANDOM_MAX,
which points to either LONG_MAX or RAND_MAX. Additionally, on 64 bit
systems holds LONG_MAX >> RAND_MAX, which leads to wrong results in
rn_uniform_zero_to_one() and rn_uniform_minusone_to_one().
Since OS X 10.7 IPPROTO_SCTP is defined in netinet/in.h
Autoconf - check for features instead of checking for host systems

Since it is likely that new OS versions implement new socket options
it is always a good idea to check for features (here socket options)
instead of checking for host systems. Add an autoconf check for
IP_MTU_DISCOVER, TCP_CORK, and TCP_CONGESTION.
Autoconf - ease check for cpuset_t by using AC_CHECK_DECL