d/rules: Export all hardening flags.

debian/rules

@@ -1,7 +1,13 @@
 #!/usr/bin/make -f
 
+export DEB_BUILD_MAINT_OPTIONS = hardening=+all
+
 %:
 	dh $@ --sourcedirectory=Src
 
+override_dh_auto_build:
+	$(MAKE) -C Src CFLAGS="$(CPPFLAGS) $(CFLAGS)" \
+		COPTS="$(CPPFLAGS) $(LDFLAGS)"
+
 # just use dh_install* instead of upstream's "make install"
 override_dh_auto_install: