build: add a feature test for linux/net_namespace.h

This header was only introduced in 2015, so we need to build without it.

travis-ci: success for series starting with [v10,01/11] net: pass the struct nlattrs to dump() functions
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

Makefile.config

@@ -18,7 +18,7 @@ export DEFINES += $(FEATURE_DEFINES)
 export CFLAGS += $(FEATURE_DEFINES)
 
 FEATURES_LIST	:= TCP_REPAIR STRLCPY STRLCAT PTRACE_PEEKSIGINFO \
-	SETPROCTITLE_INIT MEMFD TCP_REPAIR_WINDOW
+	SETPROCTITLE_INIT MEMFD TCP_REPAIR_WINDOW NET_NAMESPACE_H
 
 # $1 - config name
 define gen-feature-test

criu/net.c

 #include <unistd.h>
 #include <sys/socket.h>
 #include <linux/netlink.h>
-#include <linux/net_namespace.h>
 #include <linux/rtnetlink.h>
 #include <linux/netfilter/nfnetlink.h>
 #include <linux/netfilter/nfnetlink_conntrack.h>
@@ -37,6 +36,13 @@
 #include "protobuf.h"
 #include "images/netdev.pb-c.h"
 
+#ifdef CONFIG_HAS_NET_NAMESPACE_H
+#include <linux/net_namespace.h>
+#else
+#define NETNSA_NSID	1
+#define NETNSA_FD	3
+#endif
+
 #ifndef IFLA_LINK_NETNSID
 #define IFLA_LINK_NETNSID	37
 #endif

scripts/feature-tests.mak

@@ -88,3 +88,14 @@ int main(int argc, char *argv[], char *envp[])
 }
 
 endef
+
+define FEATURE_TEST_NET_NAMESPACE_H
+
+#include <linux/net_namespace.h>
+
+int main(int argc, char **argv)
+{
+        return 0;
+}
+
+endef