- stdlib.h
- stddef.h
- stdint.h
- stdarg.h
- limits.h
- float.h

`gettimeofday` emulation needed no longer.

On configurations where `sizeof(mrb_int) > sizeof(int)`.

The issue was reported by @shuujii

`gmtime_r` detection logic was too strict.

The size and signedness of `mrb_int` and `time_t` may differ.

This reverts commit dc51d89a.

Instead of including `mruby/presym.h` everywhere, we provided the
fallback `mruby/presym.inc` under `include/mruby` directory, and specify
`-I<build-dir>/include` before `-I<top-dir>/include` in `presym.rake`.
So even when someone drops `-I<build-dir>/include` in compiler options,
it just compiles without failure.

Addressed an issue where existing programs linking `libmruby.a` could only
be built by adding `<build-dir>/include` to compiler's include path.

|        Previous Name         |        New Name         |
|------------------------------|-------------------------|
| MRB_ENABLE_ALL_SYMBOLS       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_SYMBOLL_ALL       | MRB_USE_ALL_SYMBOLS     |
| MRB_ENABLE_CXX_ABI           | MRB_USE_CXX_ABI         |
| MRB_ENABLE_CXX_EXCEPTION     | MRB_USE_CXX_EXCEPTION   |
| MRB_ENABLE_DEBUG_HOOK        | MRB_USE_DEBUG_HOOK      |
| MRB_DISABLE_DIRECT_THREADING | MRB_NO_DIRECT_THREADING |
| MRB_DISABLE_STDIO            | MRB_NO_STDIO            |
| ENABLE_LINENOISE             | MRB_USE_LINENOISE       |
| ENABLE_READLINE              | MRB_USE_READLINE        |
| DISABLE_MIRB_UNDERSCORE      | MRB_NO_MIRB_UNDERSCORE  |
| DISABLE_GEMS                 | MRB_NO_GEMS             |

* `MRB_ENABLE_SYMBOLL_ALL` seems to be a typo, so it is fixed.
* `MRB_` prefix is added to those without.
* The previous names can also be used for compatibility.

`mrb_int()` includes implicit integer conversion, where `mrb_integer()`
does not. In this case, we know `obj` is an integer before hand.

Where fixnum overflow can happen.

- Integrate `Fixnum` and `Integer`
- Remove `Integral`
- `int / int -> int`
- Replace `mrb_fixnum()` to `mrb_int()`
- Replace `mrb_fixnum_value()` to `mrb_int_value()`.
- Use `mrb_integer_p()` instead of `mrb_fixnum_p()`

We still have `#define MRB_TT_FIXNUM MRB_TT_INTEGER` for compatibility.

- `MRB_WITHOUT_FLOAT` => `MRB_NO_FLOAT`
- `MRB_USE_FLOAT` => `MRB_USE_FLOAT32`

The former is to use `USE_XXX` naming convention. The latter is to make
sure `float` is 32bit float and not floating point number in general.

On platforms where `time_t` is unsigned, negative time can be a result
of integer casting. Out-of-range error is too strict for those cases.
This fix does not address wrong time value in `MRB_WORD_BOXING`. It will
be addressed later (by introducing "big" integers).

`mrb_get_arg1()` raises `ArgumentError` if the method does not receive one
argument.

And replaces all `mrb_get_args(mrb, "o", &arg)` by the new function.

'stdio.h' is included in 'mruby.h' ('mrbconf.h').

However, keep 'stdio.h' used by mruby-test.

Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:260:55: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
    if ((MRB_INT_MAX > MRB_TIME_MAX && i > 0 && i > MRB_TIME_MAX) ||
                                                  ^
  ```

Silence the following warnings:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:871:15: warning: result of comparison of constant 9223372036854775807 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->sec > MRB_INT_MAX || tm->sec < MRB_INT_MIN) {
        ~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:871:40: warning: result of comparison of constant -9223372036854775808 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->sec > MRB_INT_MAX || tm->sec < MRB_INT_MIN) {
                                 ~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:887:16: warning: result of comparison of constant 9223372036854775807 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->usec > MRB_INT_MAX || tm->usec < MRB_INT_MIN) {
        ~~~~~~~~ ^ ~~~~~~~~~~~
  /mruby/mrbgems/mruby-time/src/time.c:887:42: warning: result of comparison of constant -9223372036854775808 with expression of type 'time_t' (aka 'long') is always false [-Wtautological-constant-out-of-range-compare]
    if (tm->usec > MRB_INT_MAX || tm->usec < MRB_INT_MIN) {
                                ~~~~~~~~ ^ ~~~~~~~~~~~
  ```

Silence the following warning:

  ```
  /mruby/mrbgems/mruby-time/src/time.c:258:60: warning: result of comparison of constant -9223372036854775808 with expression of type 'mrb_int' (aka 'int') is always false [-Wtautological-constant-out-of-range-compare]
        if ((mrb_time_int)i > MRB_TIME_MAX || MRB_TIME_MIN > i) {
                                              ~~~~~~~~~~~~ ^ ~
  ```

- `Fixnum#to_s`, `Fixnum#inspect`
- `Float#to_s`, `Float#inspect`
- `NilClass#to_s`, `NilClass#inspect`
- `FalseClass#to_s`, `FalseClass#inspect`
- `TrueClass#to_s`, `TrueClass#inspect`
- `Time#to_s`, `Time#inspect`