Extract compiler-specific macro definitions in a new wx/compiler.h.

This solves the problem with wx/defs.h -> wx/platform.h -> wx/setup.h which
resolves to msvc/wx/setup.h -> wx/version.h -> wx/cpp.h include path which
resulted in __VISUALC__ not being defined in wx/cpp.h.

This problem was not new but went unnoticed for a long time and was only
discovered when wxCHECK_VISUALC_VERSION() started being used in wx/cpp.h too
as now the compiler started warning about wrong #if syntax due to it being
undefined.

Putting the compiler-specific definitions in a separate file allows this file
to be included from wx/cpp.h to ensure that these symbols are always defined
in it and also makes things a little better organized.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@74496 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/cpp.h

@@ -13,6 +13,8 @@
 #ifndef _WX_CPP_H_
 #define _WX_CPP_H_
 
+#include "wx/compiler.h"    /* wxCHECK_XXX_VERSION() macros */
+
 /* wxCONCAT works like preprocessor ## operator but also works with macros */
 #define wxCONCAT_HELPER(text, line) text ## line
 
@@ -130,7 +132,7 @@
     #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 199901L) || \
         (defined(__cplusplus) && __cplusplus >= 201103L)
         #define HAVE_VARIADIC_MACROS
-    #elif defined(__GNUC__) && __GNUC__ >= 3
+    #elif wxCHECK_GCC_VERSION(3,0)
         #define HAVE_VARIADIC_MACROS
     #elif wxCHECK_VISUALC_VERSION(8)
         #define HAVE_VARIADIC_MACROS