Refactor CFLAGS handling to allow non clang/gcc compilers.

The Solaris Studio compiler uses different flags...

gtests/net/packetdrill/Makefile.Darwin

 packetdrill-ext-libs := -lpcap
-CFLAGS = -D__APPLE_USE_RFC_3542
+CFLAGS = -D__APPLE_USE_RFC_3542 -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
 LDFLAGS =
 ifneq ("$(wildcard /usr/lib/libsctp.dylib)","")
     packetdrill-ext-libs += -lsctp

gtests/net/packetdrill/Makefile.FreeBSD

 packetdrill-ext-libs := -lprocstat -lutil -lkvm -lelf -lpthread -lpcap
+CFLAGS = -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
 .include "Makefile.common"

gtests/net/packetdrill/Makefile.Linux

 packetdrill-ext-libs := -lpthread -lrt -ldl -lsctp -static
+CFLAGS = -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
 include Makefile.common

gtests/net/packetdrill/Makefile.NetBSD

 packetdrill-ext-libs := -lpthread -lpcap
+CFLAGS = -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
 .include "Makefile.common"

gtests/net/packetdrill/Makefile.OpenBSD

 packetdrill-ext-libs := -lpthread -lpcap
+CFLAGS = -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
 .include "Makefile.common"

gtests/net/packetdrill/Makefile.common

 all: binaries
 
-CFLAGS += -g -Wall -Werror -Wno-unknown-warning-option -Wno-address-of-packed-member
+CFLAGS += -g
 
 parser.o: parser.y
 	bison --output=parser.c --defines=parser.h --report=state parser.y
@@ -8,7 +8,7 @@ parser.o: parser.y
 
 lexer.o: lexer.l parser.o
 	flex -olexer.c lexer.l
-	$(CC) -O2 -g -Wall -Wno-unknown-warning-option -Wno-address-of-packed-member -c lexer.c
+	$(CC) -O2 $(CFLAGS) -c lexer.c
 
 packetdrill-lib := \
          checksum.o code.o config.o hash.o hash_map.o ip_address.o ip_prefix.o \