Don't add invalid content to static lib INTERFACE_LINK_LIBRARIES.

Only valid target names or generator expressions may appear in
the target field of a LINK_ONLY expression.

Other content like link flags should still be added to that property
(wrapped in config-specific generator expressions), but not wrapped
in LINK_ONLY. Otherwise undue warnings would be issued for the
policy CMP0022.

The LINK_ONLY expression only has an effect for actual target
names anyway, so there is no logical deficit.
Merge topic 'fix-tll-static-private' into next

25f724c Don't add invalid content to static lib INTERFACE_LINK_LIBRARIES.