Update C++ standard specified for protobuf build

[riftEmber]

Recent versions of protobuf depend on abseil-cpp, and abseil requires at least C++14. However, we specify the use of C++11 in the compile flags for our protoc-gen-chpl target. Update it to C++17 since that's what Chapel in general requires.

d1a649a (#16817) asserts the -std argument is required for building on Mac, which holds up on my Mac with Clang 15; however, it works with -std=c++17 instead of 11.

[reviewer info placeholder]

Testing:

• make protoc-gen-chpl succeeds with a protobuf requiring abseil, on Mac and Linux
  • not on the latest protobuf however, due to https://github.com/Cray/chapel-private/issues/6313, but that's a separate issue