Respect the CFLAGS passed on us and don't overwrite them.

--- Makefile.orig	2007-11-27 01:57:27.000000000 +0100
+++ Makefile	2008-05-21 13:49:36.000000000 +0200
@@ -13,8 +13,8 @@
 CC=gcc
 # What a pity, all new gccs are buggy and -Werror does not work. Sigh.
 #CCOPT=-D_GNU_SOURCE -O2 -Wstrict-prototypes -Wall -g -Werror
-CCOPT=-D_GNU_SOURCE -O2 -Wstrict-prototypes -Wall -g
-CFLAGS=$(CCOPT) $(GLIBCFIX) $(DEFINES) 
+CCOPT?=-D_GNU_SOURCE -O2 -Wstrict-prototypes -Wall -g
+CFLAGS?=$(CCOPT) $(GLIBCFIX) $(DEFINES) 
 
 IPV4_TARGETS=tracepath ping clockdiff rdisc arping tftpd rarpd
 IPV6_TARGETS=tracepath6 traceroute6 ping6