Improve the logic added by commit f0e67da061 (target_link_libraries: Fix
out-of-dir linking of a list of targets, 2020-01-14, v3.17.0-rc1~149^2)
to, while exporting targets, look up referenced target names in the
correct directory.
In a case like
target_link_libraries(targetInOtherDir PUBLIC "$<1:a;b>")
then all entries in the list need to be looked up in the caller's
scope. Previously our `::@(directory-id)` suffix would apply only
to the last entry. Instead surround the entire entry by a pair
`::@(directory-id);...;::@` so that the `::@` syntax can encode
a directory lookup scope change evaluated as the list is processed.
Fixes: #20204
Previously the command did not allow naming targets on the LHS that
were not created in the calling directory. Lift this restriction to
enable more flexible use by projects.
Targets named on the RHS will need to be looked up during generation in
the scope of the call site rather than the scope of the LHS target.
Introduce an internal syntax in `[INTERFACE_]LINK_LIBRARIES` properties
to specify target names that need to be looked up in a directory other
than that containing the target on which the property is set. Add
minimal documentation of the syntax to help users that encounter it.
Unfortunately CMake previously did allow such calls in the case that
only `INTERFACE` libraries are specified, but those libraries would be
looked up in the target's directory rather than the caller's. Add
policy `CMP0079` to enable the new behavior with new lookup scope in a
compatible way.
Fixes: #17943
Brad King