Bump CMake minimum requirement to 2.8.0

[C. Boemann]

On Saturday 11 August 2012 10:15:05 Gopalakrishna Bhat wrote:
> Hi All,
> 
> Considering that CMake 2.8.0[1] was released 3 years back, I propose that
> we update our minimum requirement of cmake to 2.8.0. Even our build
> server[2] uses cmake 2.8.
> 
> This bumping is also made important by the fact that we already use
> VERSION_GREATER, VERSION_EQUAL comparisons which was introduced only in
> 2.8.0[3]
It has my support :)