add_subdirectory: Fix EXCLUDE_FROM_ALL on directory with an interface library
Since !2816 (merged) we automatically forward the EXCLUDE_FROM_ALL
to targets as they are created. This regressed support for interface libraries on which the property is not allowed.
Skip forwarding the EXCLUDE_FROM_ALL
property for interface libraries. It is not needed on them because they do not participate in the generated build system anyway.
Fixes: #18896 (closed)