Fix libtiff compilation with VC6 after upgrade to 4.0.3.

VC6 headers don't define INVALID_SET_FILE_POINTER constant so do it ourselves
if necessary.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@73242 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/tiff/libtiff/tif_win32.c

@@ -32,6 +32,11 @@
 
 #include <windows.h>
 
+/* This define is missing from VC6 headers. */
+#ifndef INVALID_SET_FILE_POINTER
+#define INVALID_SET_FILE_POINTER ((DWORD)-1)
+#endif
+
 static tmsize_t
 _tiffReadProc(thandle_t fd, void* buf, tmsize_t size)
 {