Give the compiler a chance to clean its temporary files when requesting its version at the configure time.

* 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.

* This function will then be used by lazy netcdf implementation to map
  a page of inaccessible memory.

* The original test did not work for clang.

* It is unclear if MPI I/O can make use of this, but it cannot hurt either.

* This version is still sub-optimal and transfers more data amongst the
  I/O server processes than strictly necessary.