Fix core dependencies resolving in dependencies gems.

Uncommented the Visual Studio version check 

Otherwise, `{"a": 1}` will cause `SyntaxError`.

Depend mruby-io and mruby-pack in mruby core

Fix possible memory access error.

include io.h on mingw

Suggested by @mattn.

Shouldn't use tempnam

add File#mtime

mruby-time: support time_t is uint

It was default on Linux. Abandoned for 2 reasons:

* some cross-platform compiler defines `__linux__` even on bare metal
  environment (RTL8196 (MIPS big endian soc) for example).

* some compilers do not align functions pointers so that we need to
  specify `-falign-functions=n` (where n>1). It's not suitable for
  default configuration.

By our measurement, `mrbtest` consumes 400KB less memory. So if your
target machine is memory tight, add the following command-line options
to `GCC` (unfortunately `clang` does not support `-falign-functions`).

`-falign-functions=2 -DMRB_METHOD_TABLE_INLINE`

Should be true for close_on_exec flag

When your target machine is big endian, specify as following in the
`build_config.rb`:

```ruby
MRuby::CrossBuild.new('32bit') do |conf|
  toolchain :gcc

  conf.big_endian
end
```

On some platform and timezone it is a valid time spec.

mruby-time: Fix mruby specific timegm() cannot return minus