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 - add check for tcp_ca_state
[Coding style] improve structure of configure.ac
Autoconf - fix a warning while checking for clock_get_time
Autoconf - double quote #includes
Autoconf - rename checks for socket options
Autoconf - add check for struct tcp_info