* This facilitates per-call selection of the underlying exchanger.

* Some platforms do not use the POSIX exit code, but at least have the
  decency to give a textual report we can use in the test framework.
* Some totally fail to report this on their own. For those systems a
  work-around was implemented.

* Also ensure that defective installations are diagnosed when possible,
  even when the user does not care.

* This follows the example of the Fortran test.

* Also remove unneeded VALUE attribute.

* This prevents a useless load from memory.