cmake 2.6 is old at this point and I'd be greatly surprised if anyone is actually using it or testing against it. It's probably time to upgrade to something approaching modern. Plus:
- Mac OS X already requires 3.0
- If HADOOP-14667 gets committed, Windows bumps to 3.1
- There is special handling in at least one CMakeLists.txt for versions less than 3.1
Given the last two points, I'd propose making the minimum 3.1, if not something higher due to compiler support for newer compilers across all platforms.