diff --git a/SystemTools.cxx b/SystemTools.cxx
index 661d651c6ec852735cea03afb6ec85bbbf7e08df..dba0dc3d983a62c13bb463b88cc6912d0bf8d6e4 100644
--- a/SystemTools.cxx
+++ b/SystemTools.cxx
@@ -631,7 +631,9 @@ bool SystemTools::FileExists(const char* filename)
 {
 #ifdef _MSC_VER
 # define access _access
-# define F_OK 0
+#endif
+#ifndef F_OK
+#define F_OK 0
 #endif
   if ( access(filename, F_OK) != 0 )
     {