Allow custom CFLAGS and CPPFLAGS in Makefile

This is consistent with the fact that custom LDFLAGS are already allowed, and
for once, it makes warning options easier to control.

Also use automatic variables $@ and $^ consistently.

Makefile

@@ -3,9 +3,7 @@ VERSION = 0.6
 PREFIX = /usr/local
 MANPREFIX = $(PREFIX)/man
 
-#CPPFLAGS = -DDEBUG
-#CFLAGS = -g
-CFLAGS = -O3 -march=native
+CFLAGS += -O3 -march=native
 LDLIBS = -lcurses
 
 DISTFILES = nnn.c config.def.h nnn.1 Makefile README.md LICENSE
@@ -21,8 +19,8 @@ $(LOCALCONFIG): config.def.h
 $(SRC): $(LOCALCONFIG)
 
 $(BIN): $(SRC)
-	$(CC) $(CFLAGS) -o $@ $(SRC) $(LDFLAGS) $(LDLIBS)
-	strip $(BIN)
+	$(CC) $(CFLAGS) $(CPPFLAGS) -o $@ $^ $(LDFLAGS) $(LDLIBS)
+	strip $@
 
 install: all
 	mkdir -p $(DESTDIR)$(PREFIX)/bin