Sebastian Holtermann authored Jan 25, 2019 and Brad King committed Jan 28, 2019

Use a dedicated `std::set` for implicit include directories instead
of (ab)using the emitted variable.  This in combination with lambdas
makes it easier to comprehend which paths are emitted.

For the implicit include directories we used to omit the
`CMAKE_SYSROOT_COMPILE`/`CMAKE_SYSROOT` prefix.  This has been changed and
the implicit paths are returned prefixed.  Implicit include directory returning
is only ever used by QtAutoGen which requires prefixed paths.  QtAutoGen passes
the (implicit) include paths to the `moc` which isn't aware of
`CMAKE_SYSROOT_COMPILE`/`CMAKE_SYSROOT`.