If only a static library is built, the user of the library that just
tries to link to the lib using the flags provided by pkg-config
might not know that only a static lib exists and that he'd have to
pass --static to pkg-config to get the internal dependencies to
be able to link the library.

For a shared build, the internal dependencies are kept in Libs.private
as before.

This matches how libav's pkg-config files are generated.