Double colon in target name means ALIAS or IMPORTED target.
CMake 2.8.12 and lower allowed the use of targets and files with double colons in target_link_libraries, with some buildsystem generators.
The use of double-colons is a common pattern used to namespace IMPORTED targets and ALIAS targets. When computing the link dependencies of a target, the name of each dependency could either be a target, or a file on disk. Previously, if a target was not found with a matching name, the name was considered to refer to a file on disk. This can lead to confusing error messages if there is a typo in what should be a target name.
The OLD behavior for this policy is to search for targets, then files on disk, even if the search term contains double-colons. The NEW behavior for this policy is to issue a FATAL_ERROR if a link dependency contains double-colons but is not an IMPORTED target or an ALIAS target.
This policy was introduced in CMake version 3.0. CMake version |release| warns when the policy is not set and uses OLD behavior. Use the cmake_policy command to set it to OLD or NEW explicitly.