Jörg Bornemann authored Jan 14, 2020 and Brad King committed Jan 28, 2020

The XXX_autogen targets are implemented as utility commands, which
means they always run, even if there weren't any changes.

For the Ninja generator and Qt >= 5.15 we're taking a different
approach: This commit adds custom commands that create
XXX_autogen/timestamp files. Those custom commands have a depfile
assigned that is generated from the depfiles that were created by moc.

The XXX_autogen targets merely wrap the XXX_autogen/timestamp custom
commands.

Fixes: #18749