MAP_STACK is not defined for Open BSD, OS X (android?)

[BurntSushi]

This was available in libc 0.1, but it looks like it got lost in the move to 0.2. It looks like it's defined for some other platforms currently though: https://github.com/rust-lang-nursery/libc/search?utf8=%E2%9C%93&q=MAP_STACK

cc @danburkert

[alexcrichton]

It looks like this isn't defined on OSX, forgotten from android, and this also appears to be missing from OpenBSD, so closing in favor of #44.

There may have been other mistakes though where constants were forgotten in the transition, though, and please let me know if you find any!