support override CFLAGS and CXXFLAGS

src/Makefile

@@ -18,13 +18,13 @@ BIN=smartdns
 OBJS_LIB=lib/rbtree.o lib/art.o lib/bitops.o lib/radix.o lib/conf.o
 OBJS=smartdns.o fast_ping.o dns_client.o dns_server.o dns.o util.o tlog.o dns_conf.o dns_cache.o http_parse.o $(OBJS_LIB)
 CFLAGS +=-O2 -g -Wall -Wstrict-prototypes -fno-omit-frame-pointer -Wstrict-aliasing 
-CFLAGS +=-Iinclude
-CFLAGS += -DBASE_FILE_NAME=\"$(notdir $<)\"
+override CFLAGS +=-Iinclude
+override CFLAGS += -DBASE_FILE_NAME=\"$(notdir $<)\"
 ifdef VER
-CFLAGS += -DSMARTDNS_VERION=\"$(VER)\"
+override CFLAGS += -DSMARTDNS_VERION=\"$(VER)\"
 endif
 CXXFLAGS=-O2 -g -Wall -std=c++11 
-CXXFLAGS +=-Iinclude
+override CXXFLAGS +=-Iinclude
 ifeq ($(STATIC), yes)
 LDFLAGS += -lssl -lcrypto -Wl,--whole-archive -lpthread -Wl,--no-whole-archive -ldl -static
 else