mctools/mcpl: MCPL release 1.4.0

Major update to CMake code, which first and foremost makes it possible for downstream CMake-based projects to use find_package( MCPL ) and through that get access to variables MCPL-related variables and build targets (i.e. the MCPL::mcpl target for projects needing to include the mcpl.h header file and link against the compiled MCPL library) (cf. github issue #58). Related to this the minimal CMake version is increased from CMake-3.0.0 to CMake-3.10.0. Another change is that Python scripts now refer to python3 rather than python (cf. github issue #55). Users on ancient systems where any of these changes cause problems are simply recommended to stick with MCPL v1.3.2 for now, as they are otherwise functionally equivalent. Reports about issues not thought to be caused by outdated software are gratefully received as always at https://github.com/mctools/mcpl/issues