The string returned is "<OpenGL version> Mesa <Mesa version>". The string in the sample code which I posted above is what is returned by the r300 driver: "1.5 Mesa 7.6-devel". This means that the supported OpenGL version is 1.5, provided by a Mesa 7.6 development version.
This is absolutely parsed correctly by both the atoi code I posted and the code that GLEE uses.
Note that I'm talking about the string returned by glGetString(GL_VERSION). This is the final supported OpenGL version, which has nothing to do with the GLX version or with how the OpenGL version is negotiated conceptually between client and server.
Edit: Oh, and there may be some strings in glxinfo which suggest something like Mesa supports OpenGL 2.1 (I can't test right now, not at home) - which is true, but completely beside the point. As long as the hardware driver only supports OpenGL 1.5, OpenGL 1.5 is what you will get, and what the version string correctly tells you. If you try to call 2.x functions, your program will crash. So again, everybody has always parsed the OpenGL version string like this and it has always been correct. I'm really curious where this misconception comes from.