* This also allows for overrides at make invocation.

* This was needed to get working builds on Piz Daint, libtool 2.4.2 generated
  bogus dependencies on libquadmath.so.

* The approach with cpp relies on GNU tools, i.e. is not portable.

* To the F77 and F2003 interface generators that is.

* And fallback to serial if not. Unfortunately this can only be queried at
  run-time it seems.

* Since write, mmap and a number of other functions are not part of
  ISO C (i.e. strictly conforming programs must not use those), we need
  to ensure the build environment provides POSIX conformance.
* This mostly prevents unrelated bug reports later on.
* From POSIX 2008, CDI unconditionally only uses getline and has a
  work-around in case that is missing.

* Do not create unnecessary temporaries.

* The previous implementation was not thread safe (accidental SAVE
  attribute) and handled string arguments of full length without a
  c_null_char terminator incorrectly.