Update for Cargo's platform-specific dependencies

[tomaka]

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.

[brendanzab]

Awesome! Could you also change the README?

[tomaka]

I'm getting errors related to -fPIC when trying to build on Linux, but I don't see how this could be related to this PR.

[bvssvni]

@tomaka I had problems with -fPIC before, perhaps you find something useful #90

[brendanzab]

cc. @cmr

[brendanzab]

@metajack Can we update glfw to the latest development version? I think this might allow this to pass.

[bvssvni]

Is this PR outdated?

[tomaka]

When I submitted this PR, we didn't have build scripts, gcc, pkgconfig, etc.
So yes it is outdated.

The best thing to do is probably to add a links = "glfw" entry in the glfw-sys package, and add in the README something similar to what I did for freetype-sys: https://github.com/PistonDevelopers/freetype-sys#for-windows-users