py-protobuf3: install failures for macOS 10.8 through 10.11: compilation error related to PROTOBUF_MAYBE_CONSTEXPR MapFieldBase(ConstantInitialized)

[mascguy (Christopher Nielsen)]

It looks like this port is failing to install for some macOS releases, blocking builds for a fair number of downstream ports:

/usr/bin/clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -pipe -Os -arch x86_64 -isysroot/ -I. -I../src -I/opt/local/Library/Frameworks/Python.framework/Versions/3.9/include/python3.9 -c google/protobuf/pyext/map_container.cc -o build/temp.macosx-10.10-x86_64-3.9/google/protobuf/pyext/map_container.o -Wno-write-strings -Wno-invalid-offsetof -Wno-sign-compare -Wno-unused-variable -std=c++11 -Wno-shorten-64-to-32 -Wno-deprecated-register -stdlib=libc++  -Wno-shorten-64-to-32
In file included from google/protobuf/pyext/map_container.cc:39:
../src/google/protobuf/map_field.h:332:37: error: constexpr constructor never produces a constant expression [-Winvalid-constexpr]
  explicit PROTOBUF_MAYBE_CONSTEXPR MapFieldBase(ConstantInitialized)
                                    ^
../src/google/protobuf/map_field.h:335:9: note: non-literal type 'internal::WrappedMutex' cannot be used in a constant expression
        mutex_(GOOGLE_PROTOBUF_LINKER_INITIALIZED),
        ^
1 error generated.
error: command '/usr/bin/clang' failed with exit code 1
Command failed:  cd "/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_python_py-protobuf3/py39-protobuf3/work/protobuf-3.14.0/python" && /opt/local/Library/Frameworks/Python.framework/Versions/3.9/bin/python3.9 setup.py --no-user-cfg --cpp_implementation install --prefix=/opt/local/Library/Frameworks/Python.framework/Versions/3.9 --root=/opt/local/var/macports/build/_opt_bblocal_var_buildworker_ports_build_ports_python_py-protobuf3/py39-protobuf3/work/destroot 
Exit code: 1
Error: Failed to destroot py39-protobuf3: command execution failed

Ignore the fact that this port is compiling code during phase destroot, as that's covered by issue:56534.

These specific failures are occurring on macOS 10.8, 10.9, 10.10, and 10.11.