* PGI pgc++ was not detected correctly.

* Later commits will fix the underlying issues.

* These variables are checked via the JSON files in PIO control setups
  and should not change type unless necessary. Consistency is not a
  necessity.

* And add corresponding check to test suite.

* 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.

This reverts commit de09e3fb.