Both set_source_files_properties() and set_property(SOURCE) now accept
two new optional arguments: DIRECTORY and TARGET_DIRECTORY.
The DIRECTORY option takes a list of relative or absolute paths
pointing to processed source directories (add_subdirectory was
already called on them).
These paths specify directory scopes where the source file properties
will be set. Previously the scope was always the currently processed
source directory.
Similarly TARGET_DIRECTORY takes a list of targets, whose source
directories will be used as the list of scopes where to set the
source file properties.
get_property() and get_source_file_property() also get the same
new arguments, except only one value can be specified instead
of a list.
Fixes: #20128
