[dhcp6relay] Make sure CFLAGS is appended to instead of overwritten

Also, use LDFLAGS when linking dhcp6relay. These two changes make sure
the flags set in environment variables (by dpkg-buildflags) are honored.

Also, explicitly enable all hardening flags in dpkg-buildflags for
dhcp6relay and dhcpmon. The change from the default set of flags is that
during linking, immediate binding of symbols is done instead of lazy
binding.

Signed-off-by: Saikrishna Arcot <[email protected]>

src/dhcp6relay/Makefile

@@ -5,7 +5,7 @@ MKDIR := mkdir
 CC := g++
 MV := mv
 LIBS := -levent -lhiredis -lswsscommon -pthread -lboost_thread -lboost_system
-CFLAGS  = -g -Wall -std=c++17 -fPIC -I $(PWD)/../sonic-swss-common/common
+CFLAGS += -Wall -std=c++17 -fPIE -I$(PWD)/../sonic-swss-common/common
 PWD := $(shell pwd)
 
 ifneq ($(MAKECMDGOALS),clean)
@@ -21,7 +21,7 @@ all: sonic-dhcp6relay
 sonic-dhcp6relay: $(OBJS)
 	@echo 'Building target: $@'
 	@echo 'Invoking: G++ Linker'
-	$(CC) -o $(DHCP6RELAY_TARGET) $(OBJS) $(LIBS) 
+	$(CC) $(LDFLAGS) -o $(DHCP6RELAY_TARGET) $(OBJS) $(LIBS)
 	@echo 'Finished building target: $@'
 	@echo ' '
 

src/dhcp6relay/debian/rules

@@ -1,4 +1,6 @@
 #!/usr/bin/make -f
 
+export DEB_BUILD_MAINT_OPTIONS=hardening=+all
+
 %:
 	dh $@ --parallel

src/dhcpmon/debian/rules

@@ -1,5 +1,7 @@
 #!/usr/bin/make -f
 
+export DEB_BUILD_MAINT_OPTIONS=hardening=+all
+
 DEB_CFLAGS_APPEND=-std=gnu11
 export DEB_CFLAGS_APPEND