du: update debug option setup

du: update debug option setup

Current Gnulib arranges for fts debugging if GNULIB_FTS_DEBUG
is defined, so key off that rather than off DU_DEBUG.
* src/du.c (fts_debug): Remove decl, as Gnulib does this now.
(FTS_CROSS_CHECK): Remove; all uses removed.
(FTS_DEBUG) [!GNULIB_FTS_DEBUG]: Remove.
(long_options) [GNULIB_FTS_DEBUG]: Add a ---debug option.
(du_files): Call fts_cross_check only if fts_debug and GNULIB_FTS_DEBUG.
(main): Set fts_debug if GNULIB_FTS_DEBUG, not DU_DEBUG.