make: allow setting the default network locking backend

As different Linux distributions are switching away from iptables
to nftables, this makes it easier to compile CRIU with a different
default network locking backend. Instead of changing the source
code it is now possible to select the nft backend like this:

    make NETWORK_LOCK_DEFAULT=NETWORK_LOCK_NFTABLES

Signed-off-by: Adrian Reber <areber@redhat.com>

Makefile

@@ -140,6 +140,10 @@ ifneq ($(GCOV),)
         CFLAGS          += $(CFLAGS-GCOV)
 endif
 
+ifneq ($(NETWORK_LOCK_DEFAULT),)
+	CFLAGS		+= -DNETWORK_LOCK_DEFAULT=$(NETWORK_LOCK_DEFAULT)
+endif
+
 ifeq ($(ASAN),1)
 	CFLAGS-ASAN	:= -fsanitize=address
 	export		CFLAGS-ASAN

criu/include/cr_options.h

@@ -70,7 +70,15 @@ enum NETWORK_LOCK_METHOD {
 	NETWORK_LOCK_SKIP,
 };
 
+/**
+ * CRIU currently defaults to the iptables locking backend.
+ *
+ * It is, however, possible to change this by defining
+ * NETWORK_LOCK_DEFAULT to a different value on the command-line.
+ */
+#ifndef NETWORK_LOCK_DEFAULT
 #define NETWORK_LOCK_DEFAULT NETWORK_LOCK_IPTABLES
+#endif
 
 /*
  * Ghost file size we allow to carry by default.