Fix a bunch of -Wunused-but-set-variable warnings.

core/include/fxcrt/fx_system.h

 // Copyright 2014 PDFium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
  
 // Original code copyright 2014 Foxit Software Inc. http://www.foxitsoftware.com
 
 #ifndef _FX_SYSTEM_H_
 #define _FX_SYSTEM_H_
 #define _FX_WIN32_DESKTOP_              1
 #define _FX_LINUX_DESKTOP_              4
@@ skipping 77 matching lines @@
 typedef unsigned short const*   FX_LPCWORD;
 typedef int                                             FX_INT32;
 typedef float                                   FX_FLOAT;
 typedef int                                             FX_BOOL;
 typedef int                                             FX_ERR;
 #define FX_SUCCEEDED(Status)    ((FX_ERR)(Status) >= 0)
 #define FX_FAILED(Status)               ((FX_ERR)(Status) < 0)
 typedef char                                    FX_CHAR;
 typedef char*                                   FX_LPSTR;
 typedef char const*                             FX_LPCSTR;
+#define FX_DWORD_MAX UINT_MAX
 typedef unsigned int            FX_DWORD;
 typedef unsigned int*           FX_LPDWORD;
 #if _FXM_PLATFORM_ == _FXM_PLATFORM_WINDOWS_
 typedef __int64                         FX_INT64;
 typedef unsigned __int64        FX_UINT64;
 #else
 typedef long long int           FX_INT64;
 typedef unsigned long long      FX_UINT64;
 #endif
 #if _FX_WORDSIZE_ == _FX_W64_
@@ skipping 234 matching lines @@
 
 #else  // _FXM_PLATFORM_ == _FXM_PLATFORM_WINDOWS_
 
 #if !defined(PRIuS)
 #define PRIuS "Iu"
 #endif
 
 #endif
 
 #endif