build system: do not rebuild ash and hush on any change to any .c file

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
diff --git a/libbb/appletlib.c b/libbb/appletlib.c
index f3d5301..58f1a94 100644
--- a/libbb/appletlib.c
+++ b/libbb/appletlib.c
@@ -61,7 +61,7 @@
 static const char usage_messages[] ALIGN1 = UNPACKED_USAGE;
 #else
 # define usage_messages 0
-#endif /* SHOW_USAGE */
+#endif
 
 #if ENABLE_FEATURE_COMPRESS_USAGE