Compile only necessary code in specified platforms

2 jobs for !161 with macos-splash-screen in 13 minutes and 12 seconds
detached
Status Job ID Name Coverage
  Build
passed #21917
linux kf5-qt5 qt5.12

00:10:33

34.0%
passed #21918
windows
windows kf5-qt5 qt5.13

00:08:04