Jörg Bornemann authored Aug 06, 2020 and Brad King committed Aug 10, 2020

Change the default value of `CMAKE_AUTOMOC_PATH_PREFIX` to `OFF` to
restore compatibility with behavior of CMake 3.15 and below.

C++ source files that are generated by Qt's meta object compiler (moc)
include the header file that was passed as input argument to moc. This
is usually a path relative to the source directory, for example

    #include "../../source/dir/myobject.h"

That is problematic for reproducible builds as described in #18815.
To cope with that, the target property AUTOMOC_PATH_PREFIX was
introduced in CMake 3.16 by commit d018d27c (Autogen: Add moc path
prefix generation (AUTOMOC_PATH_PREFIX), 2019-09-13, v3.16.0-rc1~94^2~4).
The property is default-initialized from the variable
`CMAKE_AUTOMOC_PATH_PREFIX`, which defaults to `ON`.

If this property is ON, and myobject.h is located in an include
directory of the target, moc-generated C++ files include the file
without the "path prefix":

    #include "myobject.h"

This behavior, h...