Allow predefining wxHAS_MODE_T to avoid mode_t conflicts

This symbol can now be predefined to avoid conflicts with mode_t definition in
wx/filefn.h when wxWidgets is used in applications also using another library
which also defines mode_t.

Closes https://github.com/wxWidgets/wxWidgets/pull/165

include/wx/filefn.h

@@ -44,7 +44,10 @@
 // constants
 // ----------------------------------------------------------------------------
 
-#if defined(__VISUALC__)
+// MSVC doesn't define mode_t, so do it ourselves unless someone else
+// had already predefined it.
+#if defined(__VISUALC__) && !defined(wxHAS_MODE_T)
+    #define wxHAS_MODE_T
     typedef int mode_t;
 #endif