Refine documentation for C99 compliance; ref #4685 [ci skip]

The refinement was based on comment from @matz.

CONTRIBUTING.md

@@ -33,9 +33,16 @@ mruby should be highly portable to other systems and compilers. For this it is
 recommended to keep your code as close as possible to the C99 standard
 (http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf).
 
-Although we target C99, Visual C++ is also an important target for mruby. For
-this reason a declaration of a local variable has to be at the beginning of a
-scope block.
+Although we target C99, we've heard some compilers in the embedded environment
+still requires declarations of local variables to be at the beginning of a
+scope. Until we confirm the situation has changed, we use the old-style
+variable declaration.
+
+Visual C++ is also an important target for mruby (supported version is 2013 or
+later). For this reason features that are not supported by Visual C++ may not
+be used (e.g. `%z` of `strftime()`).
+
+NOTE: Old GCC requires `-std=gnu99` option to enable C99 support.
 
 #### Reduce library dependencies to a minimum