Follow log_level even when using journald output

fix #3328

Only use sd_journal_* if we really are outputing to journald

We check that the device and inode numbers are actually the same as the
JOURNAL_STREAM value, instead of just checking that the value exists.

This fixes the logger unit tests

Fix the build without systemd, by adding a few define

When biboumi is logging into journald, use sd_journal_send

This makes sure that multi-lines messages are properly parsed as a single
message by journald.

fix #3268

Refactoring louloulibs and cmake

Use OBJECT libraries
Remove the louloulibs directory
Write FOUND variables in the cache

Remove all the libs that are now in louloulibs

Fix the systemd-conditional code

By using SYSTEMD_FOUND instead of SYSTEMDDAEMON_FOUND, where I forgot to
rename it…

Properly display [DEBUG] on debug lines, instead of [ERROR]

Use systemd-daemon logging prefix if available

Upgrade to C++14

Use the logger everywhere

Add a logger class