CMake Error at only-targets.cmake:[0-9]+ \(target_link_libraries\):
  Target "lib2" has LINK_LIBRARIES_ONLY_TARGETS enabled, but it links to:

    external

  which is not a target.  Possible reasons include:

    \* There is a typo in the target name.
    \* A find_package call is missing for an IMPORTED target.
    \* An ALIAS target is missing.

Call Stack \(most recent call first\):
  CMakeLists.txt:[0-9]+ \(include\)
