Makefile: automatically generate dependencies.

This uses a trick from scottmcpeak.com/autodepend/autodepend.html. It
may seem like overkill for such a small codebase, but note that header
modifications did not trigger rebuilds in the affected object files in
the existing Makefile.

.gitignore

@@ -1,2 +1,3 @@
 *.o
+*.d
 ssd1306_bin

Makefile

@@ -4,12 +4,22 @@ LDFLAGS=-static
 OBJS=main.o ssd1306.o linux_i2c.o
 BIN=ssd1306_bin
 
-%.o:%.c
-		$(CC) -c -o $@ $< $(CFLAGS)
+default: $(BIN)
+.PHONY: default clean
+
+# Adapted from scottmcpeak.com/autodepend/autodepend.html
+-include $(OBJS:.o=.d)
+%.o: %.c
+	$(CC) -c $(CFLAGS) $< -o $*.o
+	$(CC) -MM $(CFLAGS) $< > $*.d
+	@cp -f $*.d $*.d.tmp
+	@sed -e 's/.*://' -e 's/\\$$//' < $*.d.tmp | fmt -1 | \
+	  sed -e 's/^ *//' -e 's/$$/:/' >> $*.d
+	@rm -f $*.d.tmp
 
 $(BIN):$(OBJS)
 	$(CC) $(CFLAGS) -o $@ $(OBJS) $(LDFLAGS)
 
 clean:
-	rm -f ./*.o $(BIN)
+	rm -f *.o *.d $(BIN)