Add GLFW build instructions to Cargo build script
Only build rlib

This is required to keep my project's build working while I'm porting
everything to Cargo. This should be a temporary hack.
Prevent build script from confusing Cargo

It seems that re-generating link.rs in .build.sh confuses Cargo into
thinking that the code has changed and needs to be rebuilt. This commit
changes the build script to only generate link.rs, if it doesn't already
exist, thereby fixing this problem.