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

Ref #4982 #4983

The behavior is different from CRuby, but we believe this is a right
behavior for mruby, which only supports either ASCII or UTF-8
exclusively; fix #4983, ref #4982

```
$ printf '\xe3\x81' | ruby -e 'p STDIN.readchar'
"\xE3\x81"
```

```
$ printf '\xe3\x81' | mruby -e 'p STDIN.readchar'
"\xE3"
```

This fix only effective when `MRB_UTF8_STRING` is set.

- mrb_utf8len() - returns the size of a UTF-8 char (in bytes)
- mrb_utf8_strlen() - returns the length of a UTF-8 string (in char)

The bug was introduced by #4712. The `getc' problem resurrected.
It should be addressed soon.

Merged `cygwin_filename()` into `filename()`.

Validate windows by Dir testing

To prevent infinite loop on errors; reported by @shuujii

build when directories and files have spaces

Modified the build to quote filenames so that it builds when files have spaces

Fix `mrb_funcall_with_block()` uses more GC arena

If `mrb->jmp` is `NULL` and the function `mrb_funcall_with_block()` is
called, GC Arena is returned from the function with over-used.

- A normal (no global exodus) return will consume two GC Arena's.
- In the event of an exception, five GC Arena are consumed.

This patch reduces consumption in both cases to one.

Fix NULL pointer dereferences in mrb_local_variables; fix #4955

However, the behavior of `#call` on the method object of
`local_variables` is not corrected.

Improve configuration guard

ref #4576 and ref #4947

* Need MRBAPI functions without `MRB_DISABLE_STDIO`:
  * mrbgems/mruby-bin-debugger
  * mrbgems/mruby-bin-mirb
  * mrbgems/mruby-bin-mrbc
  * mrbgems/mruby-bin-mruby
  * mrbgems/mruby-bin-strip
* Need `stdio.h`:
  * mrbgems/mruby-io
  * mrbgems/mruby-print
* Need `snprintf()` in `stdio.h`:
  * mrbgems/mruby-pack
  * mrbgems/mruby-sprintf

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

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

Some rule to CrossBuild Android