RPATH: Add option for using $ORIGIN in build tree
This PR addresses #18413 (closed) by making build RPATHs relative using the
$ORIGIN token. It does not add support for macOS since that platform has quite some unusual characteristics.
CMAKE_BUILD_RPATH_USE_ORIGINby default (allowing the user/project to override it).
Introduce a new policy that disables this new feature.Deferred.
New test case: check that an external library outside the build dir does not become relative. Not sure how to do this, can I just link to
Tested on Linux only. OpenBSD and DragonFly BSD would be interesting test cases due to the