ddl: add compatibility with old versions of make

$(file ...) function is available in make only starting from 3.82.90:
http://git.savannah.gnu.org/cgit/make.git/commit/?id=fca11f60390cf607f68b497c3909b1fb40251070
Therefore, rework the file creation to call a 'file' 'function' (i.e. variable 'file') if the built-in file function is not available.