The option is not available in the old version of Ruby.

- `pool`
- `syms`
- `reps`

You have to specify `TARGET` to specify a configuration, e.g.

```
rake TARGET=host-debug all test
```

When you port `mruby` to a new configuration:
1. copy an existing configuration under `target` directory
2. modify the new configuration file
3. build using the new configuration
4. send PR if you please

* Disable automatic update and clean up on `brew install` (install time
  160 sec -> 5 sec).
* Avoid using deprecated keys.

Because some changes have been overridden.

I sometimes see Bison related problems in setting up build environments.
Therefore to remove Bison from build time dependencies, add `y.tab.c`
generated by Bison to the repository.

The reduction of dependency at build time also reduces the labor and time
for setup and installation in CI.

In addition, a path in `#line` directive is converted to a relative path so
that its path is constant regardless of development environments.

`Windows-MinGW` and `Windows-VC` also requires updates.

- `mrb_convert_type`
- `mrb_check_convert_type`

Those function no longer take `tname` string representation of desired
type, and take method symbols instead of `const char*` names. This is
incompatible change. I hope no third-party gems use those functions.

- mrb_define_class_id
- mrb_define_module_id
- mrb_define_method_id
- mrb_define_singleton_method_id
- mrb_define_module_function_id
- mrb_define_const_id
- mrb_undef_method_id
- mrb_undef_class_method_id
- mrb_class_defined_id
- mrb_class_get_id
- mrb_class_defined_under_id
- mrb_class_get_under_id
- mrb_module_get_id
- mrb_module_get_under_id
- mrb_define_class_under_id
- mrb_define_module_under_id
- mrb_exc_get_id

- :

Except for support files e.g. `mruby-test/driver.c`, which are not
target of symbol collection via `rake gensym`.

`mrb_funcall_id()` takes `mrb_sym` instead of `char*` for a method name.
You can use `MRB_SYM()`/`MRB_QSYM()` to specify the method to call.

You don't need to invoke `rake gensym` explicitly any longer.

`libmruby.flags.mak` is only required from `mruby-config` gem.

Where `QSYM` means quoted symbols, which cannot be represented C
symbols, so specify aliases instead.

- operators: name of the operation, e.g. add for `+`
- predicates: add `_p` suffix instead of `?`
- bang methods: add `_b` suffix instead of `!`
- instance variables: add `a_` prefix instead of `@`
- global variables: add `d_` prefix instead of `@`
- class variables: unsupported; don't use them

For example, `MRB_OPSYM(add)` refers a symbol for `+`.

- MRB_PRESYM_CSYM
- MRB_PRESYM_SYM