Fix $MACOSX_DEPLOYMENT_TARGET mismatch

kokoro/macos/python_cpp/build.sh

@@ -9,4 +9,5 @@ cd $(dirname $0)/../../..
 source kokoro/macos/prepare_build_macos_rc
 g++ --version
 
+export MACOSX_DEPLOYMENT_TARGET=10.9
 ./tests.sh python_cpp