A couple sendmmsg() portability fixes

The first one makes fixes sendmmsg() runtime detection as to make it available to RHEL 6.2 and possibly others. It's untested on that platform, but worked on tests of mine on both 2.6.32 (negative) & 3.2.0 (positive).

The second one I expect to be a little more controversial. The goal is to make the sendmmsg() functionality available in certain systems (*cough* Debian wheezy) but in the expense of ugly preprocessor clutter & hacks.