Suppress SGI MIPSpro warning 3968

The warning appears everywhere we use static_cast to explicitly truncate
an integer width.  It appears in the form

  cc-3968 CC: WARNING File = ..., Line = ...
    implicit conversion of a 64-bit integral type to a smaller
    integral type (potential portability problem)
      static_cast<...>(...);
      ^

which is strange because a "static_cast" is not implicit.  It also
appears in system library code.

CTestCustom.cmake.in

@@ -39,7 +39,7 @@ SET(CTEST_CUSTOM_WARNING_EXCEPTION
   "warning:.*is.*very unsafe.*consider using.*"
   "warning:.*is.*misused, please use.*"
   "CMakeSetupManifest.xml.*manifest authoring warning.*Unrecognized Element"
-  "cc-3968 CC: WARNING File.*/usr/include/CC/fstream"
+  "cc-3968 CC: WARNING File.*" # "implicit" truncation by static_cast
   )
 
 IF(NOT "@CMAKE_GENERATOR@" MATCHES "Xcode")