Fix Makefile to respect user variables

--- a/Makefile
+++ b/Makefile
@@ -3,9 +3,8 @@
 # by default, you probably need to add -I/usr/whereever/flac to CFLAGS and
 # -L/usr/lib/whereever to the link step.
 
-CFLAGS="-Wall -g"
-
 flac-image: flac-image.o
-	gcc -o flac-image -lFLAC flac-image.o
+	$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $^ -lFLAC
 
-flac-image: flac-image.c flac-image.h
\ No newline at end of file
+flac-image.o: flac-image.c flac-image.h
+	$(CC) $(CFLAGS) $(CPPFLAGS) -c -o $@ $<