Add wxNO_UNSAFE_WXSTRING_CONV2 macro

The macro disallows implicit conversions between wxString and const
char*

include/wx/anystr.h

@@ -65,6 +65,7 @@ public:
     bool operator!() const { return !((bool)*this); }
 
 
+#ifndef wxNO_IMPLICIT_WXSTRING_ENCODING
     // and these are the conversions operator which allow to assign the result
     // of FuncReturningAnyStrPtr() to either char* or wxChar* (i.e. wchar_t*)
     operator const char *() const
@@ -94,6 +95,7 @@ public:
 
         return p;
     }
+#endif // wxNO_IMPLICIT_WXSTRING_ENCODING
 
     operator const wchar_t *() const
     {