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* This bug was introduced in 5862f0a3.

* Extend intersection xmap test, so similar bugs cannot creep in as
  easily again.

* Since the MPI datatype construction machinery of YAXT does not
  depend on the other parts of YAXT, it's sufficient to check on a
  higher level only.

* This way, the constructor cannot ever be invoked if the MPI version is < 3.

* Setting XT_CONFIG_DEFAULT_EXCHANGE_METHOD allows for an adjusted default.