Mesa 12.0 Might Be A Delayed Release This Summer With OpenGL 4.5 Support
David Airlie made a proposal this weekend to hold off on Mesa 12.0 until OpenGL 4.5 support can be announced for at least some hardware. More than likely, the hardware target for OpenGL 4.5 in Mesa 12.0 would be Intel Skylake graphics.
The bulk of the remaining work is robustness, cull_distance, enhanced_layouts, and SIMD32 for i965 compute shaders. You can read this thread on the Mesa mailing list about this call for delaying the release and focusing on Intel Skylake OpenGL 4.5 support.
Other upstream Mesa developers so far have jumped on board with this proposal, while adding they should cap it as a one month maximum delay. If in the extra month they can clear OpenGL 4.5 support, then everything would be dandy, otherwise they'd just the release as-is. Ilia Mirkin also mentioned that Nouveau's Fermi/Kepler support might also soon be into shape for OpenGL 4.5 compliance.
Of course, on MesaMatrix.net you can get a quick overview of what needs to be done GL4 wise for each of the different drivers.