Fixes for BSD/MacOSX and array limits

I tried to use sngrep on mac os x 10.9, the issues were:
* no stdbuf - even after installing coreutils via macports - so, I went for: configure --disable-ngrep
* there was an error related to posix threads, which I got it fixed with first patch in this pull request
* clang reported warnings about going over the limits of declared arrays, which seemed legit -- second patch is for fixing them
* another compile warning was fixed by the third patch

stdbuf tool doesn't seem to be on mac, found and alternative here https://github.com/tcreech/stdbuf-osx -- maybe I will give it a try later.