boost configured with -DBOOST_INCLUDE_LIBRARIES=headers does not install headers

I'm trying to install only the boost headers using the cmake build system. My impression was that this could be achieved by configuring with ```cmake -DBOOST_INCLUDE_LIBRARIES=headers```. However, this seems to result in installing only the cmake files, but not the headers.

In direct comparison, using b2, ```bootstrap --with-libraries=headers``` works as expected.

In detail:

```
wget https://github.com/boostorg/boost/releases/download/boost-1.92.0/boost-1.92.0-cmake.tar.xz
tar xf boost-1.92.0-cmake.tar.xz
cd boost-1.92.0
cmake -DBOOST_INCLUDE_LIBRARIES=headers .
make install DESTDIR=/tmp
```

Full output of make install:

```
Install the project...
-- Install configuration: "Release"
-- Up-to-date: /tmp/usr/local/include
-- Up-to-date: /tmp/usr/local/include/boost
-- Up-to-date: /tmp/usr/local/include/boost/headers
-- Up-to-date: /tmp/usr/local/include/boost/headers/.gitkeep
-- Installing: /tmp/usr/local/lib/cmake/boost_headers-1.92.0/boost_headers-targets.cmake
-- Installing: /tmp/usr/local/lib/cmake/boost_headers-1.92.0/boost_headers-config.cmake
-- Installing: /tmp/usr/local/lib/cmake/boost_headers-1.92.0/boost_headers-config-version.cmake
-- Up-to-date: /tmp/usr/local/lib/cmake/Boost-1.92.0/BoostConfig.cmake
-- Installing: /tmp/usr/local/lib/cmake/Boost-1.92.0/BoostConfigVersion.cmake
```

Reproduce the issue with `cmake -DBOOST_INCLUDE_LIBRARIES=headers .` followed by `make install DESTDIR=/tmp`, then trace how the `headers` selection controls installation. Done means the Boost headers are installed along with the generated CMake package files, matching the b2 `--with-libraries=headers` behavior.

Written by the indexing model from the issue text.