Update for rustc

[tomaka]

This first commit fixes the ICE, but I've got another problem behind:

src/gl/lib.rs:88:1: 96:2 error: expected ident, found `$`
src/gl/lib.rs:88 generate_gl_bindings! {
src/gl/lib.rs:89     api: "gl",
src/gl/lib.rs:90     profile: "core",
src/gl/lib.rs:91     version: "4.5",
src/gl/lib.rs:92     generator: "global",
src/gl/lib.rs:93     extensions: [

[tomaka]

The error comes from here: https://github.com/bjz/gl-rs/blob/678346eb5f8ba95f50a8d0f7ff9d87a7338953dc/src/gl_generator/generators/global_gen.rs#L161

[tomaka]

Someone should open an issue in rust-lang/rust because of this poor interaction between quote_item! and static mut (see the second commit and the error in the OP).
I don't have the motivation to test this.

The ICE has already been reported here: rust-lang/rust#18763