Also on macOS Mojave on a new mac, I had the same issue, but macOS_SDK_headers_for_macOS_10.14.pkg wasn’t there. I got it by downloading and running this from the developer site. Running xcode-select --install might also have worked.
Then I had to fix some tests but then the compile completed.
Slicer runs, but it’s not usable. This is with Qt 5.11.2 on a macbook pro retina, with the resolution set as “best for display”.
