iproute2: Auto-detect the presence of setns in libc

If libc has setns present use that version instead of
rolling the syscall wrapper by hand.

Dan McGee found the following compile error:

    gcc -D_GNU_SOURCE -O2 -Wstrict-prototypes -Wall -I../include
    -DRESOLVE_HOSTNAMES -DLIBDIR=\"/usr/lib/\"   -c -o ipnetns.o ipnetns.c
    ipnetns.c:31:12: error: static declaration of ‘setns’ follows non-static
    declaration
    /usr/include/bits/sched.h:93:12: note: previous declaration of ‘setns’
    was here
    make[1]: *** [ipnetns.o] Error 1

Reported-by:  Dan McGee <dan@archlinux.org>
Tested-by:  Dan McGee <dan@archlinux.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

configure

@@ -158,6 +158,27 @@ check_ipt_lib_dir()
 	echo "not found!"
 }
 
+check_setns()
+{
+cat >/tmp/setnstest.c <<EOF
+#include <sched.h>
+int main(int argc, char **argv) 
+{
+	(void)setns(0,0);
+	return 0;
+}
+EOF
+gcc -I$INCLUDE -o /tmp/setnstest /tmp/setnstest.c >/dev/null 2>&1
+if [ $? -eq 0 ]
+then
+	echo "IP_CONFIG_SETNS:=y" >>Config
+	echo "yes"
+else
+	echo "no"
+fi
+rm -f /tmp/setnstest.c /tmp/setnstest
+}
+
 echo "# Generated config based on" $INCLUDE >Config
 
 echo "TC schedulers"
@@ -173,3 +194,6 @@ check_ipt
 
 echo -n "iptables modules directory: "
 check_ipt_lib_dir
+
+echo -n "libc has setns: "
+check_setns

ip/Makefile

@@ -7,6 +7,12 @@ IPOBJ=ip.o ipaddress.o ipaddrlabel.o iproute.o iprule.o ipnetns.o \
 
 RTMONOBJ=rtmon.o
 
+include ../Config
+
+ifeq ($(IP_CONFIG_SETNS),y)
+	CFLAGS += -DHAVE_SETNS
+endif
+
 ALLOBJ=$(IPOBJ) $(RTMONOBJ)
 SCRIPTS=ifcfg rtpr routel routef
 TARGETS=ip rtmon

ip/ipnetns.c

@@ -28,6 +28,7 @@
 #define MNT_DETACH	0x00000002	/* Just detach from the tree */
 #endif /* MNT_DETACH */
 
+#ifndef HAVE_SETNS
 static int setns(int fd, int nstype)
 {
 #ifdef __NR_setns
@@ -37,6 +38,7 @@ static int setns(int fd, int nstype)
 	return -1;
 #endif
 }
+#endif /* HAVE_SETNS */
 
 
 static int touch(const char *path, mode_t mode)