We represent stub libraries, e.g., for CUDA, using imported `SHARED`
library targets with only `IMPORTED_IMPLIB`, and no `IMPORTED_LOCATION`,
to indicate that the stub file is meant only for linkers and not dynamic
loaders. See commit 7351d590ee (cmTarget: Add a way to represent
imported shared library stubs, 2023-07-17, v3.28.0-rc1~344^2) and commit
fc6508921c (cmComputeLinkInformation: Restore soname lookup for
non-imported targets, 2023-12-05, v3.28.0~4^2).
If a shared library is linked to a stub, it has a `NEEDED` field
populated with the `SONAME` found in the stub. When a dependent target
links to such a shared library, some linkers want to find a library file
on disk and load it to see what symbols it provides. This is necessary
for linkers that enforce `--no-allow-shlib-undefined`. On hosts with
only the stub library installed, e.g., with only the CUDA toolkit
development package, the real runtime library corresponding to the
stub's `SONAME` may not even exist, so no `-rpath-link` flag can help
linkers find it. Pass the stub library to linkers explicitly so they
can find it without searching.
This makes binaries independent of the build directory by not embedding
the build directory via RPATH. The tests are partially based on the
existing RuntimePath test, but with the check moved into a POST_BUILD
command such that it can be skipped when the platform lacks support.
Fixes: #18413