A meaningful value of X might come about if you select packages that implement the various parts of POSIX, which defines a unix-like operating system environment.
In between the libraries, syscalls and utilities, I think one would basically come up with what would amount to GNU+Linux.
What is a useful operating systems these days is a valid question. Are windowing systems included ? Desktop environments ? All the other useful software that has become indispensible to most but are outside of the scope of the specification?
But as far as a formally defined POSIX compliant operating system environment goes it's should be reasnoable to say that "What makes Linux UNIX is GNU, which is not UNIX."
Ofcourse, RMS must have framed the posix debate in the first place, since he named it, so this metric doesn't count :)