done a dozen of randconfig test. guess what? ALL failed...
these are resulting fixes
diff --git a/coreutils/diff.c b/coreutils/diff.c
index a49d519..072e4bc 100644
--- a/coreutils/diff.c
+++ b/coreutils/diff.c
@@ -71,7 +71,7 @@
 char *start, *label[2];
 struct stat stb1, stb2;
 char **dl;
-static int dl_count;
+USE_FEATURE_DIFF_DIR(static int dl_count;)
 
 struct cand {
 	int x;