Add rb-sys-env to ease integration pains

[ianks]

Why

So after working to make rb-sys seamless with magnus, there are a number of integration pain points I've stumbled across:

1. Cargo link directives do not propagate transitively

This means that means the link-args set by rb-sys do not automatically propagated to magnus. This can cause unexpected breakage (especially on windows)

More info: (rust-lang/cargo#9554)

2. There is some baseline information that's needed at compile time

This like the ruby_gte_3_0, and ruby_use_flonum are examples of this. Exposing this information in an easy way would alleviate a lot of headaches.

How

This PR adds a lightweight, dependency-free rb-sys-env crate to glue high level bindings crates together properly. IMPORTANT: This crate is only needed if you are wrapping rb-sys to make a high-level bindings crate (i.e. this is not needed if you are a gem author)!

At it's core, all it does is properly set cargo configuration based on the DEP_RB_* environment variables which are already set by rb-sys. This means this crate does not have to depend on rb-sys directly, and as such builds quickly.

Usage

In your high-level Ruby library that wraps rb-sys

1. Add this to your Cargo.toml:

   [build-dependencies]
   rb-sys-env = "0.1"

2. Then, just add this your build.rs:

   pub fn main() -> Result<(), Box<dyn std::error::Error>> {
     let _rb_env = rb_sys_env::activate()?;
   
     Ok(())
   }

What it configures

Anytime you want to see what is being set by rb-sys-env, you can run RB_SYS_ENV_DEBUG=1 cargo test...

👉 Click here to see an example output

  cargo:rustc-cfg=ruby_use_unaligned_member_access
  cargo:rustc-cfg=ruby_have_rb_ext_ractor_safe
  cargo:rustc-cfg=ruby_have_ruby_missing_h
  cargo:rustc-cfg=ruby_have_ruby_re_h
  cargo:rustc-cfg=ruby_have_ruby_debug_h
  cargo:rustc-cfg=ruby_have_ruby_oniguruma_h
  cargo:rustc-cfg=ruby_have_rb_io_t
  cargo:rustc-cfg=ruby_use_rincgc
  cargo:rustc-cfg=ruby_have_rb_data_type_t_function
  cargo:rustc-cfg=ruby_have_rb_data_type_t_parent
  cargo:rustc-cfg=ruby_have_ruby_intern_h
  cargo:rustc-cfg=ruby_have_ruby_defines_h
  cargo:rustc-cfg=ruby_have_ruby_thread_native_h
  cargo:rustc-cfg=ruby_have_rb_scan_args_optional_hash
  cargo:rustc-cfg=ruby_have_ruby_ruby_h
  cargo:rustc-cfg=ruby_have_ruby_io_h
  cargo:rustc-cfg=ruby_have_ruby_fiber_scheduler_h
  cargo:rustc-cfg=ruby_have_ruby_version_h
  cargo:rustc-cfg=ruby_have_rb_define_alloc_func
  cargo:rustc-cfg=ruby_use_transient_heap
  cargo:rustc-cfg=ruby_have_ruby_encoding_h
  cargo:rustc-cfg=ruby_have_ruby_random_h
  cargo:rustc-cfg=ruby_have_ruby_vm_h
  cargo:rustc-cfg=ruby_have_ruby_st_h
  cargo:rustc-cfg=ruby_have_ruby_ractor_h
  cargo:rustc-cfg=ruby_have_ruby_thread_h
  cargo:rustc-cfg=ruby_have_ruby_onigmo_h
  cargo:rustc-cfg=ruby_use_symbol_as_method_name
  cargo:rustc-cfg=ruby_have_rb_fd_init
  cargo:rustc-cfg=ruby_have_ruby_util_h
  cargo:rustc-cfg=ruby_have_ruby_regex_h
  cargo:rustc-cfg=ruby_have_rb_reg_new_str
  cargo:rustc-cfg=ruby_use_mjit
  cargo:rustc-cfg=ruby_use_flonum
  cargo:rustc-cfg=ruby_use_rgengc
  cargo:rustc-cfg=ruby_have_ruby_atomic_h
  cargo:rustc-cfg=ruby_have_ruby_memory_view_h
  cargo:rustc-cfg=ruby_3
  cargo:rustc-cfg=ruby_3_1
  cargo:rustc-cfg=ruby_3_1_2
  cargo:rustc-cfg=ruby_gte_2_2
  cargo:rustc-cfg=ruby_gt_2_2
  cargo:rustc-cfg=ruby_gte_2_3
  cargo:rustc-cfg=ruby_gt_2_3
  cargo:rustc-cfg=ruby_gte_2_4
  cargo:rustc-cfg=ruby_gt_2_4
  cargo:rustc-cfg=ruby_gte_2_5
  cargo:rustc-cfg=ruby_gt_2_5
  cargo:rustc-cfg=ruby_gte_2_6
  cargo:rustc-cfg=ruby_gt_2_6
  cargo:rustc-cfg=ruby_gte_2_7
  cargo:rustc-cfg=ruby_gt_2_7
  cargo:rustc-cfg=ruby_gte_3_0
  cargo:rustc-cfg=ruby_gt_3_0
  cargo:rustc-cfg=ruby_lte_3_1
  cargo:rustc-cfg=ruby_3_1
  cargo:rustc-cfg=ruby_eq_3_1
  cargo:rustc-cfg=ruby_gte_3_1
  cargo:rustc-cfg=ruby_lt_3_2
  cargo:rustc-cfg=ruby_lte_3_2
  cargo:rustc-cfg=ruby_lt_3_3
  cargo:rustc-cfg=ruby_lte_3_3
  cargo:rustc-cfg=ruby_gte_1
  cargo:rustc-cfg=ruby_gt_1
  cargo:rustc-cfg=ruby_gte_2
  cargo:rustc-cfg=ruby_gt_2
  cargo:rustc-cfg=ruby_lte_3
  cargo:rustc-cfg=ruby_3
  cargo:rustc-cfg=ruby_eq_3
  cargo:rustc-cfg=ruby_gte_3
  cargo:rustc-cfg=ruby_lt_4
  cargo:rustc-cfg=ruby_lte_4
  cargo:rustc-link-search=native=/Users/ianks/.rubies/3.1.2/lib
  cargo:rustc-link-lib=ruby.3.1
  cargo:rustc-link-arg=-Wl,-rpath,ruby.3.1
  cargo:rustc-link-arg=-Wl,-multiply_defined,suppress