Explicitly check for netinet/ip_icmp.h, since
it is avaliable on Mac OS X, but not on iOS.
This allows building the stack without IP/IPv6
support.
This fixes the issue reported in
https://code.google.com/p/sctp-refimpl/issues/detail?id=7


git-svn-id: http://sctp-refimpl.googlecode.com/svn/trunk/KERN/usrsctp/usrsctplib@9167 9df1edf5-d72c-5b5f-11c0-5f5209eb73f7