`CMAKE_AUTOMOC_RELAXED_MODE` was added for backwards compatibility with KDE 4,
which had its last release in 2014.  It does not offer additional features
but complicates the `AUTOMOC` code and dependency computation considerably.

Projects that use `CMAKE_AUTOMOC_RELAXED_MODE` functionality always got
extensive warnings during builds and tips on how to convert to regular mode,
which is trivial (see commit e474dcb2, CMake 2.8.7).

It's time to consider this feature deprecated and issue a warning at
configuration time as well.

This adds a configuration time deprecation `AUTHOR_WARNING` for
`CMAKE_AUTOMOC_RELAXED_MODE`.