Define wxHAS_NULLPTR_T if nullptr is supported

This is necessary because MSVS versions as old as 2010 support it, even
if they don't have full C++11 support and so don't define __cplusplus
appropriately.

Extract the existing check for nullptr from wx/strvararg.h to wx/defs.h
in order to allow reusing it.

include/wx/defs.h

@@ -285,6 +285,14 @@ typedef short int WXTYPE;
     #define wxOVERRIDE
 #endif /*  HAVE_OVERRIDE */
 
+/*
+    Support for nullptr is available since MSVS 2010, even though it doesn't
+    define __cplusplus as a C++11 compiler.
+ */
+#if __cplusplus >= 201103 || wxCHECK_VISUALC_VERSION(10)
+    #define wxHAS_NULLPTR_T
+#endif
+
 /* wxFALLTHROUGH is used to notate explicit fallthroughs in switch statements */
 
 #if __cplusplus >= 201703L