This project is using the phrase "system call" idiosyncratically / incorrectly: by it they mean "calls to the system() function", not "low-level OS facilities". POCO and ACE seem to be about actual system calls (network programming, threads, pipes, etc.) They're for very different purposes.

libcox is for when your existing code would do something like system("uname") or system("ls") (... except not even with system(), with popen() or something), and you want it portable to Windows.