devel/cmake: prefer python3 over versioned python3.x

Depending on which mechanism is used to find Python (Python3
in particular, since there's only one Python2 version left),
either python3 is found -- installed by the python3 metaport,
and presumed to be the "preferred / default Python3 version" --
of a versioned python3. Asking for, say, Python 3.6 will get
you whatever python3 points at -- assuming it is newer than 3.6 --
or, with the other mechanism, the newest python (e.g. highest
installed version). Those are not necessarily the same.

Switch the new mechanism to preferring python3; there's a
knob in CMake itself for that, we just need to set it in
the FreeBSD config-file.

PR: 252277