CI: semantic version. CMake DRY refactor

[scivision]

CI: avoid nuisance warnings about outdated libraries by using semantic versions in *.yml

CMake: DRY (don't repeat yourself) by refactoring to inspect target sources and set CXX property in one place.

CMake: avoid nuisance extra CTest targets and use consistent variable name NINJA_BUILD_TESTING by using enable_testing()

[jhasse]

Hm ... isn't BUILD_TESTING supposed to be the clean solution to disable one's tests? Introducing our own variable sounds like a bad idea and not like the clean CMake way (not that there is one).

[scivision]

Some projects choose to use ${PROJECT_NAME}_BUILD_TESTING and others use BUILD_TESTING. I'll switch it to BUILD_TESTING to avoid disruption.