Pass CPPFLAGS and LDFLAGS to the compiler so the build respects the
user's flags.

--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,7 @@
 CC=gcc

 all: zerofree.c
-	$(CC) $(CFLAGS) -o zerofree zerofree.c -lext2fs
+	$(CC) $(CFLAGS) $(CPPFLAGS) $(LDFLAGS) -o zerofree zerofree.c -lext2fs

 clean:
 	/bin/rm -f *.o *~ zerofree