MinGW symbol visibility fix

When using separate sources MinGW would use GCC symbol visibility attributes
which cause: "warning: visibility attribute not supported in this
configuration; ignored".

Avoid using GCC symbol visibility attributes with MinGW.  Instead, use either
no visibility attributes or Windows specific __declspec(dllexport) and
__declspec(dllimport).

src/duk_features.h.in

@@ -1966,8 +1966,11 @@ typedef FILE duk_file;
 /* XXX: separate macros for function and data may be necessary at some point. */
 
 #if defined(DUK_F_GCC_VERSION)
-#if (DUK_F_GCC_VERSION >= 40000)
-/* Might work on earlier versions too but limit to GCC 4+. */
+#if (DUK_F_GCC_VERSION >= 40000) && !defined(DUK_F_MINGW)
+/* Might work on earlier versions too but limit to GCC 4+.
+ * MinGW should use Windows specific __declspec or no visibility attributes at all,
+ * otherwise: "warning: visibility attribute not supported in this configuration; ignored"
+ */
 #define DUK_F_GCC_SYMBOL_VISIBILITY
 #endif
 #endif