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)