I'm not sure it's as clear which is the primary definition as you're implying.  The metre is defined as the distance travelled by light in vacuum in \frac{1}{299792458} s, defining the speed of light as 299792458 m/s. That's a practical definition relating to actual photons in a real-world vacuum.  From this perspective, relativity shows that the universal speed limit is equal to the speed that photons actually travel at, rather than vice versa.