Check for POSIX.1-2001 conformance.
* 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.