add -std=c++11

Author: skvark

setup.py

@@ -137,6 +137,8 @@ def main():
     if sys.platform == 'darwin':
         cmake_args.append("-DWITH_LAPACK=OFF")  # Some OSX LAPACK fns are incompatible, see
                                                 # https://github.com/skvark/opencv-python/issues/21
+        cmake_args.append("-DCMAKE_CXX_FLAGS=-std=c++11")  # enable c++11 explicitly for clang
+
     if sys.platform.startswith('linux'):
         cmake_args.append("-DWITH_IPP=OFF")   # https://github.com/opencv/opencv/issues/10411