Remove -Wno-unknown-warning-option, which is not needed anymore.

gtests/net/packetdrill/Makefile.Darwin

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

gtests/net/packetdrill/Makefile.FreeBSD

-ARCH != uname -p
 packetdrill-ext-libs := -lprocstat -lutil -lkvm -lelf -lpthread -lpcap
 CFLAGS = -Wall -Werror
-.if ${ARCH} != "powerpc" && ${ARCH} != "powerpc64"
-  CFLAGS += -Wno-unknown-warning-option
-.endif
 .include "Makefile.common"

gtests/net/packetdrill/Makefile.Linux

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

gtests/net/packetdrill/Makefile.NetBSD

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

gtests/net/packetdrill/Makefile.OpenBSD

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