Error when using external cl headers

```
CMake Error: install(EXPORT "OpenCLHeadersCppTargets" ...) includes target "HeadersCpp" which requires target "OpenCLHeaders" that is not in any export set.
CMake Error in CMakeLists.txt:
export called with target "HeadersCpp" which requires target
"OpenCLHeaders" that is not in any export set.
```

Here's how we invoke cmake
https://github.com/conda-forge/clhpp-feedstock/blob/65ec400d4f221776e11078c5903b4c56cc8edc83/recipe/build.sh#L6-L16

Any ideas on how to fix this?

Start with the CMake invocation in the linked recipe/build.sh and inspect the repository's CMakeLists.txt. Reproduce the install(EXPORT) and export errors involving HeadersCpp and OpenCLHeaders, then determine the expected export-set relationship. Done means the external-header configuration completes without either target-export error.

Written by the indexing model from the issue text.