[CMake] CMake minimum required version

[Russell Wallace]

CMake asks me to specify a minimum required version. I'm writing a
mostly fairly vanilla C++ program, primary target platforms are Linux
and Windows, dependencies on Boost and GMP, using the glob facility,
any idea what I should specify as the minimum version? Or is the
recommendation to reason along the lines of, 2.8 is what I happen to
have installed, so that's the version I know my program works with
since I'm testing with it, so specify that version?