Fix both code paths:
- Change the test source code so it doesn't cause the "Unused variable"
  warning (which -Werror converted into error and caused CMake not to set
  HAVE_THREAD_LOCAL)

- If the system doesn't seem to support HAVE_THREAD_LOCAL, refuse to
  compile (rather than producing a binary that crashes for some tests)

Originally submitted at https://github.com/facebook/mysql-5.6/pull/905