* Also make macro use safe for clang.

* While this particular bug was fixed in PGI 20.1 and 20.4 AFAIK,
  it is back now with the 20.11 release.

* While the mismatch was strictly a bug, no Fortran compiler currently
  has default kind integer different from c_int kind.

* This way clang emits no warnings about missing declarations for
  externally visible functions.
* Also, merge improvements to core.c and core_cf.c from ppm.

* 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.