It is up to the book to manage its attribute.
Also remove the `absolutePath` (and `indexAbsolutePath`). The `Book::path` is always stored
absolute.
The fact that the path can be stored absolute or relative in the
`library.xml` is not relevant for the book.
There is no more integrated build of dependencies in the build system.
Dependencies are discovered using pkg-config except for ctpp2 where there
is no pkg-config file.