improve makefile

Makefile

@@ -1,12 +1,14 @@
 CFLAGS=-std=c99 -Wall -g -O2
 INDENTOPTS=-kr --no-tabs --braces-on-func-def-line --indent-level2
 
+TARGETS=approximate-pi linked-list mandelbrot mandelbrot.bmp threads
+
 .PHONY: all
-all: approximate-pi linked-list mandelbrot threads
+all: $(TARGETS)
 
 .PHONY: clean
 clean:
-	rm -f approximate-pi linked-list mandelbrot threads *~
+	rm -f $(TARGETS) *~
 
 .PHONY: indent
 indent:
@@ -15,5 +17,8 @@ indent:
 mandelbrot: mandelbrot.c
 	$(CC) $(CFLAGS) $(shell sdl-config --cflags) -o $@ $< $(shell sdl-config --libs) -lm
 
+mandelbrot.bmp: mandelbrot
+	./mandelbrot
+
 threads: threads.c
 	$(CC) $(CFLAGS) -o $@ $< -pthread