subj. Roughly that +shared enables shared libs, whereas ~shared implies that static-only libs will be built.

Most likely we cant really enforce +shared not to have static libs, as some packages may build them anyway.

from @alalazo

What all the other packages do is to build both shared and static libraries when +shared and just static when ~shared.

While here, also document +pic variant for static libs only and that +pic+shared should throw an error.