target_link_libraries: Allow use with targets in other directories

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

1 files changed, 5 insertions, 0 deletions

diff --git a/Tests/RunCMake/target_link_libraries/CMP0079-link-WARN-stderr.txt b/Tests/RunCMake/target_link_libraries/CMP0079-link-WARN-stderr.txt
new file mode 100644
index 0000000..0b4c4c6
--- /dev/null
+++ b/Tests/RunCMake/target_link_libraries/CMP0079-link-WARN-stderr.txt
@@ -0,0 +1,5 @@
+^CMake Error at CMP0079-link/CMakeLists.txt:[0-9]+ \(target_link_libraries\):
+  Attempt to add link library "foo" to target "top" which is not built in
+  this directory.
+
+  This is allowed only when policy CMP0079 is set to NEW.$