Updating Bindgen Instructions for Glue

[redfire75369]

I was looking to add a wrapper for JSExternalStringCallbacks so it could be used for rust, and I've updated jsglue.cpp for it.

I was wondering how I can update glue.rs since it seems to be automatically generated. I was looking at the instructions in comments in gen.py but it seems to be very outdated.

# To generate jsglue:
# DYLD_LIBRARY_PATH=~/versioned/rust-mozilla/build/llvm/x86_64-apple-darwin/Release+Asserts/lib/ ~/versioned/rust-bindgen/bindgen ./jsglue.c -I ../../build/src/mozjs/dist/include/ -match glue > glue.rs

# jdm used
# LD_LIBRARY_PATH=~/sdb/rust/build/llvm/x86_64-unknown-linux-gnu/Release+Asserts/lib/ ~/sdb/rust-bindgen/bindgen ./jsglue.c -I ../../build/src/mozjs/dist/include/ -isystem /usr/lib/gcc/x86_64-redhat-linux/4.7.0/include/ -match glue >glue.rs
# on Fedora 17
#
# and remember to mark all generated functions as #[rust_stack]

I think it would be useful if we updated this so that anyone can modify jsglue.cpp easily.

Here's the change I was trying to make: redfire75369@6893940

[jdm]

I actually strongly suspect that glue.rs has not been automatically generated for many years, and is only updated by hand.

[redfire75369]

Oh, that would certainly explain it.
It seems gen.py was last updated 10 years ago, which explains the wrong command line arguments and the usage of gcc 4.8.

[redfire75369]

So far, the furthest I've gotten is this, but that seems to maybe generate a bunch of stuff that's not needed?

// cwd: ./src
bindgen jsglue.cpp --allow-file jsglue.cpp -- -std=c++17 -I ../target/debug/build/mozjs-1d605e44d5141b22/out -I ../target/debug/build/mozjs_sys-253b4187986dd3b4/out/build/dist/include