get_current_dir_name exists on GNU libc only, so not portable.
On the other hand, while POSIX leaves a null buffer argument for getcwd
as unspecified behaviour, basically the most used/important Unix
implementations (GNU libc, FreeBSD's libc, etc) allow such value,
returning a new allocated buffer with the current directory.
In any case, the change just affects two tests, so even if it hits a
libc not implementing this behaviour for getcwd, only tests are
affected.
7fcac5f59f