Update for Cargo's platform-specific dependencies

`glfw-sys` is now built only on `i686-unknown-linux-gnu`, `x86_64_unknown_linux_gnu`, `i686-apple-darwin` and `x86_64-apple-darwin` (all relevant platforms but windows).

In the long-term this should be changed again to use the new build commands system that has been accepted but is not yet implemented in Cargo.

**Note**: I didn't test if it works on Linux because platform-specific dependencies are not in the Cargo nightlies yet. I recommend quickly trying on OS/X too.