Don’t hardcode CFLAGS and LDFLAGS into Makefile.

From: Seth M. Price <sprice623@aol.com>

--- a/Makefile
+++ b/Makefile
@@ -3,8 +3,8 @@ SRCS = $(SRC)/bridge.c $(SRC)/debug.c $(SRC)/getcmd.c $(SRC)/ip.c $(SRC)/init.c
 OBJS = $(SRC)/bridge.o $(SRC)/debug.o $(SRC)/getcmd.o $(SRC)/ip.o $(SRC)/init.o $(SRC)/modem_core.o $(SRC)/nvt.o $(SRC)/serial.o $(SRC)/ip232.o $(SRC)/util.o $(SRC)/phone_book.o $(SRC)/tcpser.o $(SRC)/dce.o $(SRC)/line.o
 CC ?= gcc
 DEF = 
-CFLAGS = -O $(DEF) -Wall
-LDFLAGS = -lpthread
+CFLAGS += 
+LDFLAGS += -lpthread
 DEPEND = makedepend $(DEF) $(CFLAGS)
 
 all:   tcpser