cmstd: Remove -isystem option for IBM i (OS400)

Much like AIX, IBM i (OS identifier "OS400") implicitly adds `extern
"C"` around system header files included with the `-isystem` option.
Update the condition added by commit c688b401d3 (cmstd: Modernize CMake
system headers, 2019-08-04, v3.16.0-rc1~81^2~1) to treat `IBM i` as we
do AIX.

CMakeLists.txt

@@ -368,7 +368,7 @@ macro (CMAKE_BUILD_UTILITIES)
   # Setup third-party libraries.
   # Everything in the tree should be able to include files from the
   # Utilities directory.
-  if (CMAKE_SYSTEM_NAME STREQUAL "AIX" AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+  if ((CMAKE_SYSTEM_NAME STREQUAL "AIX" OR CMAKE_SYSTEM_NAME STREQUAL "OS400") AND CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
     # using -isystem option generate error "template with C linkage"
     include_directories("${CMake_SOURCE_DIR}/Utilities/std")
   else()