Makefile: Fix Make.dep not updated by config changes

Make.dep file should be updated by .config changed after first make.
There are 2 cases affected for this problem:

 1) Add source files by config symbol
 2) Include header files in #ifdef directive

These 2 cases may not be included in Make.dep and this may prevent the
differential build from working correctly.

syscall/Makefile

@@ -93,7 +93,7 @@ $(BIN3): $(WRAP_OBJS)
 
 $(SYSCALLWRAPS): .context
 
-.depend: Makefile $(SRCS)
+.depend: Makefile $(SRCS) $(TOPDIR)$(DELIM).config
 	$(Q) $(MKDEP) $(PROXYDEPPATH) $(STUBDEPPATH) $(WRAPDEPPATH) \
 	  "$(CC)" -- $(CFLAGS) -- $(SRCS) >Make.dep
 	$(Q) touch $@