cmake BUILD_INTERFACE

CMake's `BUILD_INTERFACE` is a generator expression that is used to specify build-time dependencies or paths. It allows you to control the behavior of targets during the build process.

When using `BUILD_INTERFACE`, you can specify paths or include directories that are only relevant during the build phase of your project. These paths or directories will not be propagated to consuming projects when they link against your target.

For example, if you have a library target and you want to include some header files only during the build process, you can use `BUILD_INTERFACE` in conjunction with `target_include_directories()`. This ensures that the headers are only included when building your target, but not when using your library as a dependency.

Here's an example:

target_include_directories(my_library
    PUBLIC
        $<INSTALL_INTERFACE:include>
        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
)

In this example, `${CMAKE_CURRENT_SOURCE_DIR}/include` is included only during the build process, while `include` is included when using the installed library.

Note that `BUILD_INTERFACE` is evaluated during the configuration step, so any paths or dependencies specified with it should be available at configure time.