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.

Fix argument error when built with MRB_WITHOUT_FLOAT flag

fix improper test of Range#min

Fixed inproper argument of mrb_fixnum_value() called in integral_div(),
when built with MRB_WITHOUT_FLOAT flag.
Co-authored-by: taiyoslime <t@iyosli.me>
Co-authored-by: n4o847 <22975590+n4o847@users.noreply.github.com>

Prohibit string changes by "s"/"z" specifier of `mrb_get_args()`

- The `s` specifier is a string pointer obtained without performing `mrb_str_modify()`, so it cannot be changed.
- The `z` specifier cannot be changed because it is a string pointer obtained by `RSTRING_CSTR()` which returns `const char *`.

examples/mrbgems: clarify the caller

Before this commit:

  mruby -e 'CRubyExtension.ruby_method' # => A Ruby Extension
  mruby -e 'CRubyExtension.c_method'    # => A C Extension
  mruby -e 'CExtension.c_method'        # => A C Extension
  mruby -e 'RubyExtension.ruby_method'  # => A Ruby Extension

After this commit:

  mruby -e 'CRubyExtension.ruby_method' # => CRubyExtension: A Ruby Extension
  mruby -e 'CRubyExtension.c_method'    # => CRubyExtension: A C Extension
  mruby -e 'CExtension.c_method'        # => CExtension: A C Extension
  mruby -e 'RubyExtension.ruby_method'  # => RubyExtension: A Ruby Extension

Remove redundant type-check

since mrb_str_to_str() also does it.

*.d format is version-independent

Fix skipping reading file

As pointed out by @shuujii on https://github.com/mruby/mruby/pull/5079 ,
multi-path in single-line could be appear even with older-gcc or clang.

Fix *.d parsing with gcc 9.3.0

Before this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  rake aborted!
  Don't know how to build task '/home/foo/mruby/build/host/src/gc.o' (See the list of available tasks with `rake --tasks`)
  ...

After this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  CC    src/gc.c -> build/host/src/gc.o
  ...

With gcc 9.3.0 on Ubuntu 20.04, build/host/src/array.d looks like:

  /build/host/src/array.o: /src/array.c \
   /include/mruby.h /include/mrbconf.h \
   /include/mruby/common.h \
   ...
   /include/mruby/range.h \
   /src/value_array.h

and it has been parsed to:

  [
                                           # /src/array.c missing
    "/include/mruby.h /include/mrbconf.h", # incorrectly parsed
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]

After this change, *.d will be parsed correctly.

  [
    "/src/array.c",
    "/include/mruby.h",
    "/include/mrbconf.h",
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]