CPM usage conflicts if Boost is already included

[mjcaisse]

If Boost has been added via CPM via something like:

add_versioned_package(
   NAME Boost
   VERSION 1.88.0
   GITHUB_REPOSITORY "boostorg/boost"
   GIT_TAG "boost-1.88.0"
)

then there is an error:

CMake Error at /path/to/cpm_cache/mp11/bf7aae5c18316ffffb3d7e62dc3f8423b7c968b1/CMakeLists.txt:9 (add_library):
  add_library cannot create target "boost_mp11" because another target with
  the same name already exists.  The existing target is an interface library

[elbeno]

Arguably this should be addressed in the boost sub projects like mp11 which are available both in boost and standalone. From CMake's point of view the issue is that we ask for two unrelated packages that happen to define clashing targets.

We can protect against this for the case of boost since it's probably common, but it's a more general problem.

[elbeno]

See intel/cicd-repo-infrastructure#211