CPM usage conflicts if Boost is already included

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
```