refactor: improve signature of 'check_directory' sub in automake

refactor: improve signature of 'check_directory' sub in automake

* automake.in (check_directory): Take the relative directory
the directory to be checked is expected to be found into as
an optional parameter, rather than reading it from the global
variable `$relative_dir'.
(scan_autoconf_traces, check_directories_in_var): Adjust.