tools build: Add test for setns()

And provide an alternative implementation to keep perf building on older
distros as we're about to add initial support for namespaces.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Krister Johansen <kjlx@templeofstupid.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-bqdwijunhjlvps1ardykhw1i@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/build/Makefile.feature b/tools/build/Makefile.feature
index 523911f..c71a05b 100644
--- a/tools/build/Makefile.feature
+++ b/tools/build/Makefile.feature
@@ -64,7 +64,8 @@
         get_cpuid                       \
         bpf                             \
         sched_getcpu			\
-        sdt
+        sdt				\
+        setns
 
 # FEATURE_TESTS_BASIC + FEATURE_TESTS_EXTRA is the complete list
 # of all feature tests