Fix wrong check of the CMAKE_VERSION (#602)
Fix wrong check of the CMAKE_VERSION (#602)

`CMAKE_VERSION_MAJOR` is just the first component of the CMake version (e.g. 3 for 3.10.1) which compares less than `3.7.9` if it is less than 3. The proper variable to compare is `CMAKE_VERSION` which contains the full version number.
neilmacintosh/GSL