reduce length of scanbuild static analyzer test

reduce length of scanbuild static analyzer test

This was ~30mn, by far the longest run on travisCI.
That's because it re-analyzes multiple times the same files (library files notably).
It also performs actions that make no sense for the static analyzer purpose,
such as building the single-file library.

Reduced time spent in this test by reducing its scope :
just build the CLI, and obviously the library along it.
These are the only ones that really deserve to be analyzed.

Unfortunately, it still results in a number of false positives when using newer versions of scanbuild
(each version of scanbuild generates a different list of false positives).
These will have to be fixed before transfering to Github Actions.