Trigger rebuild on configuration change

Pass configuration macros via an autogenerated header file instead of
compiler command line options. So now dependecies tracking could
trigger rebuild on configuration option change.

This should facilitate packaging for such distro as OpenWrt, which keeps
unpacked sources and compiled object files and so needs for rebuild
trigger on configuration change.

Author: rsa9000

.gitignore

@@ -1,3 +1,5 @@
 atheepmgr
 *.d
 *.o
+.__config
+config.h

Makefile

@@ -61,14 +61,30 @@ DEPFLAGS=-MMD -MP
 
 all: $(TARGET)
 
-$(TARGET): $(OBJ)
-	$(CC) $^ $(LDFLAGS) -o $@
+FORCE:
+
+$(TARGET): config.h $(OBJ)
+	$(CC) $(OBJ) $(LDFLAGS) -o $@
 
 %.o: %.c
-	$(CC) $(DEPFLAGS) $(CFLAGS) $(DEFS) -c $< -o $@
+	$(CC) $(DEPFLAGS) $(CFLAGS) -include config.h -c $< -o $@
+
+.__config: FORCE
+	@echo "$(DEFS)" | tr ' ' '\n' | grep -v '^$$' > $@.tmp
+	@diff $@ $@.tmp > /dev/null 2>&1 && rm -f $@.tmp || mv $@.tmp $@
+
+config.h: .__config
+	@echo "Generate config.h"
+	@echo '/* Automatically generated. DO NOT EDIT. */' > $@.tmp
+	@echo '#ifndef _CONFIG_H_' >> $@.tmp
+	@echo '#define _CONFIG_H_' >> $@.tmp
+	@sed -re 's/-D([A-Z0-9_]+)/#define \1/' $^ >> $@.tmp
+	@echo '#endif' >> $@.tmp
+	@mv $@.tmp $@
 
 clean:
 	rm -rf $(TARGET)
+	rm -rf .__config config.h
 	rm -rf $(OBJ)
 	rm -rf $(DEP)