DJGPP compilation fixes

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@13032 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/string.cpp

@@ -133,7 +133,7 @@ extern const wxChar WXDLLEXPORT *wxEmptyString = &g_strEmpty.dummy;
     #endif //compiler
 #endif // no vsnprintf
 
-#ifdef _AIX
+#if defined(_AIX) || defined(__DJGPP__)
   // AIX has vsnprintf, but there's no prototype in the system headers.
   extern "C" int vsnprintf(char* str, size_t n, const char* format, va_list ap);
 #endif