Make build system respect user flags

--- a/Makefile
+++ b/Makefile
@@ -1,6 +1,4 @@
-CC=gcc
 RM=rm -f
-CFLAGS=-ggdb -O3 -W -Wall
 
 C_SOURCES=alac.c \
           demux.c \
@@ -11,10 +9,10 @@
 OBJECTS=$(C_SOURCES:.c=.o)
 
 alac: $(OBJECTS)
-	$(CC) -o alac $(OBJECTS)
+	$(CC) $(LDFLAGS) $(CFLAGS) -o alac $(OBJECTS)
 
 %.o: %.c
-	$(CC) -c $(CFLAGS) $< -o $@
+	$(CC) $(CFLAGS) $(CPPFLAGS) -c $< -o $@
 
 clean:
 	rm -f $(OBJECTS) *.*~ alac