Use 2 digits exponent format for printf family on old MSVC

For conforming C standard.

src/state.c

@@ -41,6 +41,10 @@ mrb_open_core(mrb_allocf f, void *ud)
 
   mrb_init_core(mrb);
 
+#if !defined(MRB_DISABLE_STDIO) && defined(_MSC_VER) && _MSC_VER < 1900
+  _set_output_format(_TWO_DIGIT_EXPONENT);
+#endif
+
   return mrb;
 }