Well, not for CM. It is unknown whether the calls that are wrongly or not at all implemented in AMD's new drivers are all in this Mesa library.
But the point is - since it is open source you could fix it yourself, unlike the AMD Windows drivers.
Chances are that the Mesa drivers are indeed more complete than AMD's new drivers because some people have been running CMx2 on Wine without complaints, and that uses the Mesa backend on AMD and Intel graphics hardware. Also, it works for X-Plane, which is also an old OpenGL game (or rather, their extensions).