Check that configured paths exist

[Jokymon]

When trying to create a new project with non-existent paths in kits.json, CMake will fail with seemingly unrelated errors about missing a make-program. In my case the problem was, that the compiler path for MinGW was wrong and the make executable wasn't added to PATH.

At least when creating a new project, cptb should check that the paths in kits.json actually exist and inform the user when that is not the case.

Alternatively or additionally it should be possible to run some kind of subcommand that makes plausability checks on all the settings.