GitLab

* This will enable saving MPI_Type_dup calls later on, where
  ownership can be simply transferred.

* 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.