SystemTools: Fix FileExists for some SCO OpenServer file permissions

Take changes used by pkgsrc [1]:

* SCO OpenServer 5.0.7/3.2's command has 711 permission.

[1] http://cdn.netbsd.org/pub/pkgsrc/current/pkgsrc/devel/cmake/patches/

Change-Id: Ib9fd7611e2c0b3e10a75b2322c002a8cb266ea89

SystemTools.cxx

@@ -1320,9 +1320,14 @@ bool SystemTools::FileExists(const std::string& filename)
   return (GetFileAttributesW(
             SystemTools::ConvertToWindowsExtendedPath(filename).c_str())
           != INVALID_FILE_ATTRIBUTES);
+#else
+// SCO OpenServer 5.0.7/3.2's command has 711 permission.
+#if defined(_SCO_DS)
+  return access(filename.c_str(), F_OK) == 0;
 #else
   return access(filename.c_str(), R_OK) == 0;
 #endif
+#endif
 }
 
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