Bump CMake minimum requirement to 2.8.0

[Cyrille Berger Skott]

On Saturday 18 Aug 2012, Gopalakrishna Bhat wrote:
> code exists. On seeing the git history I see that this was added to enable
> "make benchmark". Any idea what is the purpose of the above code snippet?
> and is this option still being used by anyone?
not entirely sure since I copy/paste it from the KDE4_ADD_UNIT_TEST macro, my 
guess is that replacing "buildtests" by "buildbenchmarks" should do the trick.

-- 
Cyrille Berger Skott