Autogen: Add moc path prefix generation (AUTOMOC_PATH_PREFIX)

The new `AUTOMOC_PATH_PREFIX` boolean target property enables automatic
generation of the path prefix `-p` option for `moc`.
`AUTOMOC_PATH_PREFIX` is initialized from the variable
`CMAKE_AUTOMOC_PATH_PREFIX`, which is ON by default.

When `AUTOMOC_PATH_PREFIX` is enabled, CMake tests if a `moc`ed header file is
in one of the include directories.  If so, then the `-p` option, with the
relative path of the header parent directory to the respective include
directory, is added to the `moc` command.  If the header file is not in an
include directory, the `-p` option is omitted.

Closes: #18815 "AUTOMOC: generated files include full relative path,
                breaking certain reproducible builds"

Modules/CMakeGenericSystem.cmake

@@ -26,6 +26,7 @@ set(CMAKE_FIND_LIBRARY_SUFFIXES ".so" ".a")
 
 set(CMAKE_AUTOGEN_ORIGIN_DEPENDS ON)
 set(CMAKE_AUTOMOC_COMPILER_PREDEFINES ON)
+set(CMAKE_AUTOMOC_PATH_PREFIX ON)
 set(CMAKE_AUTOMOC_MACRO_NAMES "Q_OBJECT" "Q_GADGET" "Q_NAMESPACE")
 
 # basically all general purpose OSs support shared libs