Extend the change made by commit fe4b25ec2f (Teach target_* commands to
set INTERFACE properties of IMPORTED targets, 2017-09-18,
v3.11.0-rc1~433^2~2) to work with imported targets of type `UNKNOWN`.
Fixes: #19434
The following was disallowed:
add_library(iface INTERFACE)
target_link_libraries(iface PUBLIC)
just due to the mention of the `PUBLIC` keyword. Instead, only error if
there are actually `PUBLIC` dependencies specified (and analogously for
other restrictions).
Update tests to expect this new behavior.
Not all of these commands accept non-compilable (i.e., IMPORTED)
targets, so those calls are currently just commented out. If they ever
do start to accept them, the tests should be enabled.
