Cleaning up our posix definition / usage.

-) Let's not use _POSIX_SOURCE. It usually implies too much C99. _BSD_SOURCE would be the right thing to do here.
-) _BSD_SOURCE is getting deprecated by glibc, so we also have to define _DEFAULT_SOURCE under Linux.
-) accept4 and eventfd arn't as old as we may think; let's detect for it.
-) stdint.h interferes with all these definitions if included too early; let's move it down.