Autogen: Turn off moc path prefix generation by default

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 (closed). 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-rc194^24). 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, however, can break projects that have equally named header files in different include directories. As "not breaking existing projects" trumps "have reproducible builds by default" we change the default of CMAKE_AUTOMOC_PATH_PREFIX to OFF.

Also, it is now possible to pass -DCMAKE_AUTOMOC_PATH_PREFIX=ON on the CMake command line. Before, it was overridden in CMakeGenericSystem.

Fixes: #20598 (closed)
Issue: #18815 (closed)
Backport: release
Topic-rename: automoc-path-prefix-off