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>

1 files changed, 24 insertions, 0 deletions

diff --git a/configure b/configure
index 0795df85..f5c3d405 100755
--- a/configure
+++ b/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