Fix build for nightly rust

Replace fmt! with format! (and fix fail! invocations)
Add #feature to generated code instead of patching gl.rs directly
And generator should now be built by CI.

Ideally we'd also fetch gl.xml, generate gl.rs and build the result, but I'm not sure how to run generator from the .yml since the output is in a folder with platform-specific suffix (i.e. x86_64-unknown-linux-gnu).