Do not fail to compile when one of them is missing but the other one is not.

Raise an error when trying to open a database with the missing library.

see #3237

This reduces the size of the final binary by 12% by not including
symbols, and thus function bodies that have been inlined for example.

Simplifies the CMakeLists.txt a little bit

Because it would overwrite the user file when installing a new version.  And
it’s basically useless anyway, because if the file doesn’t exist at all,
biboumi will write an helpful message on stderr, telling the user to create
the config file.

fix #3260

fix #3244

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

Should compile faster (and also take less space on disk)

We don’t need any UTF-8 support here, and it’s more portable

fix #3226

fix #3211

This coverage is probably more useful than the other one, because it’s not
really reasonable to try to cover everything with simple units test (e.g. on
network code etc).
However, we should strive to have 100% coverage with our e2e tests.