Use `_Static_assert` when C11 is available and use it in pointer size check.
Check `RSTRING_EMBED_LEN_MAX` with static assert.
Fix `Time.at` args spec.
Fallback to `mrb_assert` instead of dirty trick.
Add a missing space after ","
Merge pull request #2374 from take-cheeze/timeat_fix

Fix `Time.at` args spec.
Merge pull request #2376 from kou/patch-1

Add a missing space after ","
Merge pull request #2373 from take-cheeze/static_assert

Add macro `mrb_static_assert`.