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

And now `default.gembox` includes `mruby-socket` gem.

- README.md
- CONTRIBUTING.md
- doc/limitations.md

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.

`assert()` can be completely removed when `NDEBUG` is set.

`presym` are symbols used in the C source files. `gensym` rake rule
scans the entire C source files and collect symbols referenced from
them.