Fix python ext build on kokoro (#4527)

python/setup.py

@@ -206,6 +206,8 @@ if __name__ == '__main__':
       v = float('.'.join(v.split('.')[:2]))
       if v >= 10.12:
         extra_compile_args.append('-std=c++11')
+    elif os.getenv('KOKORO_BUILD_NUMBER') or os.getenv('KOKORO_BUILD_ID'):
+        extra_compile_args.append('-std=c++11')
 
     if warnings_as_errors in sys.argv:
       extra_compile_args.append('-Werror')