Since commit c9a5f34b (Cleanup use of CMake version in install
destinations, 2013-09-26) we use the full CMake version in default
install destinations.  For rapidly changing versions of the form

 <major>.<minor>.<patch>.<date>-g<commit>[-dirty]

used by developers this is too granular.  Instead use just

 <major>.<minor>.<patch>

as the version for default install destinations.  This will still
be granular enough for unique directories in releases.

On Cygwin continue to use the full ${CMake_VERSION} as was the case
prior to the above-mentioned commit.