Headers exported from a sourceset that are applied to a binary (as opposed to component) are not available to a consuming component:

    libraries {
        hello {
            source sources.main.cpp
            binaries.all {
                source sources.linux.cpp
            }
        }
    }
    executables {
        main {
            lib libraries.hello
        }
     }

Then the headers from 'sources.main.cpp' are included when compiling 'executables.main', but headers for 'sources.linux.cpp' are not.