[!707] dune_add_library: handle object library with `SOURCES` option

Merge branch 'dune_add_library-OBJECT-SOURCES' into 'master'

ref:core/dune-common Previously the OBJECT path of dune_add_library would only
use the unparsed arguments. Source files specified via the SOURCES option were
ignored.

See merge request [!707]

  [!707]: gitlab.dune-project.org/core/dune-common/merge_requests/707