Currently errors out any clang compilation on windows with mingw-w64
as std::mutex is not set with constexpr
full error:
```c++
../../src/google/protobuf/struct.pb.cc:42:76: error: variable does not have a constant initializer
PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_NO_DESTROY StructDefaultTypeInternal _Struct_default_instance_;
^~~~~~~~~~~~~~~~~~~~~~~~~
../../src/google/protobuf/struct.pb.cc:42:1: note: required by 'require_constant_initialization' attribute here
PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_NO_DESTROY StructDefaultTypeInternal _Struct_default_instance_;
^~~~~~~~~~~~~~~~~~
../../src\google/protobuf/port_def.inc:580:30: note: expanded from macro 'PROTOBUF_CONSTINIT'
\#define PROTOBUF_CONSTINIT [[clang::require_constant_initialization]]
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
../../src\google/protobuf/stubs/mutex.h:132:17: note: non-constexpr constructor 'mutex' cannot be used in a constant expression
std::mutex mu_{};
^
../../src\google/protobuf/map_field.h:336:9: note: in call to 'WrappedMutex()'
mutex_(GOOGLE_PROTOBUF_LINKER_INITIALIZED),
^
../../src\google/protobuf/map_field.h:485:9: note: in call to 'MapFieldBase({})'
: MapFieldBase(tag) {}
^
../../src\google/protobuf/map_field.h:546:9: note: in call to 'TypeDefinedMapFieldBase({})'
: TypeDefinedMapFieldBase<Key, T>(tag), impl_() {}
^
../../src/google/protobuf/struct.pb.cc:33:5: note: in call to 'MapField({})'
: fields_(::PROTOBUF_NAMESPACE_ID::internal::ConstantInitialized{}){}
^
../../src/google/protobuf/struct.pb.cc:36:7: note: in call to 'Struct({})'
: _instance(::PROTOBUF_NAMESPACE_ID::internal::ConstantInitialized{}) {}
^
../../src/google/protobuf/struct.pb.cc:42:76: note: in call to 'StructDefaultTypeInternal()'
PROTOBUF_CONSTINIT PROTOBUF_ATTRIBUTE_NO_DESTROY StructDefaultTypeInternal _Struct_default_instance_;
^
D:\mabs\msys64\mingw64\include\c++\10.2.0\bits/std_mutex.h:91:5: note: declared here
mutex() noexcept = default;
^
1 error generated.
```
Signed-off-by: Christopher Degawa <ccom@randomderp.com>