Add wxUSE_UNICODE_UTF16 macro to represent if the internal representation of wxString is UTF-16 or not

The value of this macro is equal to (wxUSE_UNICODE_WCHAR && SIZEOF_WCHAR_T == 2).

include/wx/chartype.h

@@ -175,6 +175,12 @@
     #define wxUSE_UTF8_LOCALE_ONLY 0
 #endif
 
+#if wxUSE_UNICODE_WCHAR && SIZEOF_WCHAR_T == 2
+    #define wxUSE_UNICODE_UTF16 1
+#else
+    #define wxUSE_UNICODE_UTF16 0
+#endif
+
 /* define char type used by wxString internal representation: */
 #if wxUSE_UNICODE_WCHAR
     typedef wchar_t wxStringCharType;