Fix MacOS protoc release (#5638)

* Use set -ex
* Use -std=c++14 so -latomic can be found

kokoro/release/protoc/macos/build.sh

@@ -1,7 +1,7 @@
 #!/bin/bash
 
-set -x
-CXXFLAGS_COMMON="-DNDEBUG -mmacosx-version-min=10.9"
+set -ex
+CXXFLAGS_COMMON="-std=c++14 -DNDEBUG -mmacosx-version-min=10.9"
 
 cd github/protobuf
 ./autogen.sh