Add settings to build for Mac

I had a problem building node-libtorrent on Mac, kind of similar to [this one](https://github.com/fanatid/node-libtorrent/issues/4). Apparently, on Mac gyp ignores 'cflags_cc' and 'cflags_cc!' settings, so the build fails. 

This commit provides proper settings for Mac build. I also hit trouble with dynamic linking after the build, so added some extra settings to overcome it.