Detect depfiles only when --detect_depfiles is specified

This makes generation time ~5% faster.
diff --git a/flags.h b/flags.h
index 905d5ee..910acbf 100644
--- a/flags.h
+++ b/flags.h
@@ -25,6 +25,7 @@
 
 struct Flags {
   bool detect_android_echo;
+  bool detect_depfiles;
   bool dump_kati_stamp;
   bool enable_kati_warnings;
   bool enable_stat_logs;