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.
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