Remove support for gcc < 4

We probably could drop support for some gcc 4.x versions too, but we
definitely don't need to support gcc 3.x any longer and not doing it
simplifies the code a bit, so just assume gcc >= 4.

Closes https://github.com/wxWidgets/wxWidgets/pull/1943

include/wx/defs.h

@@ -481,10 +481,9 @@ typedef short int WXTYPE;
 
 #ifndef HAVE_WOSTREAM
     /*
-        Mingw <= 3.4 and all versions of Cygwin don't have std::wostream
+        Cygwin is the only platform which doesn't have std::wostream
      */
-    #if (!defined(__MINGW32__) || wxCHECK_GCC_VERSION(4, 0)) && \
-        !defined(__CYGWIN__)
+    #if !defined(__CYGWIN__)
         #define HAVE_WOSTREAM
     #endif
 #endif /* HAVE_WOSTREAM */
@@ -622,15 +621,11 @@ typedef short int WXTYPE;
  */
 #define wxDEPRECATED(x) wxDEPRECATED_DECL x
 
-#if defined(__GNUC__) && !wxCHECK_GCC_VERSION(3, 4)
-    /*
-        We need to add dummy "inline" to allow gcc < 3.4 to handle the
-        deprecation attribute on the constructors.
-    */
-    #define  wxDEPRECATED_CONSTRUCTOR(x) wxDEPRECATED( inline x)
-#else
-    #define  wxDEPRECATED_CONSTRUCTOR(x) wxDEPRECATED(x)
-#endif
+/*
+    This macro used to be defined differently for gcc < 3.4, but we don't
+    support it any more, so it's just the same thing as wxDEPRECATED now.
+ */
+#define wxDEPRECATED_CONSTRUCTOR(x) wxDEPRECATED(x)
 
 /*
    Macro which marks the function as being deprecated but also defines it