Fix compiler warnings

* Avoid reinterpret_cast
* Remove pointless #define _GNU_SOURCE. It is already defined by gyp,
  and defining it after all the includes does nothing anyway.
* Avoid using Nothing for primitives, they are not initialized.
* unary minus operator applied to unsigned type, result still unsigned

src/external_copy/serializer_nortti.cc

@@ -18,7 +18,7 @@ void SerializerDelegate::ThrowDataCloneError(Local<String> message) {
 
 auto SerializerDelegate::GetSharedArrayBufferId(
 		Isolate* /*isolate*/, Local<SharedArrayBuffer> shared_array_buffer) -> Maybe<uint32_t> {
-	auto result = Nothing<uint32_t>();
+	auto result = Just<uint32_t>(0);
 	detail::RunBarrier([&]() {
 		transferables.emplace_back(std::make_unique<ExternalCopySharedArrayBuffer>(shared_array_buffer));
 		result = Just<uint32_t>(transferables.size() - 1);
@@ -34,7 +34,7 @@ auto SerializerDelegate::GetWasmModuleTransferId(
 }
 
 auto SerializerDelegate::WriteHostObject(Isolate* /*isolate*/, Local<Object> object) -> Maybe<bool> {
-	auto result = Nothing<bool>();
+	auto result = Just<bool>(false);
 	detail::RunBarrier([&]() {
 		serializer->WriteUint32(transferables.size());
 		transferables.emplace_back(TransferOut(object));

src/isolate/allocator_nortti.cc

@@ -21,7 +21,7 @@ class ExternalMemoryHandle {
 		~ExternalMemoryHandle() {
 			auto* allocator = IsolateEnvironment::GetCurrent().GetLimitedAllocator();
 			if (allocator != nullptr) {
-				allocator->AdjustAllocatedSize(-size);
+				allocator->AdjustAllocatedSize(-static_cast<ptrdiff_t>(size));
 			}
 		};
 

src/isolate/environment.cc

@@ -55,7 +55,6 @@ static auto GetStackBase() -> void* {
   return base;
 }
 #elif defined __unix__
-#define _GNU_SOURCE
 static auto GetStackBase() -> void* {
 	pthread_t self = pthread_self();
 	pthread_attr_t attrs;

src/module/lib_handle.cc

@@ -54,8 +54,13 @@ auto LibHandle::Hrtime(MaybeLocal<Array> maybe_diff) -> Local<Value> {
 auto LibHandle::PrivateSymbol(MaybeLocal<String> maybe_name) -> Local<Value> {
 	Local<String> name{};
 	if (maybe_name.ToLocal(&name)) { /* nothing */ }
+	union {
+		Local<Private> *symbol;
+		Local<Value> *value;
+	} cast;
 	auto symbol = Private::New(Isolate::GetCurrent(), name);
-	return *reinterpret_cast<Local<Value>*>(&symbol);
+	cast.symbol = &symbol;
+	return *cast.value;
 }
 
 // NOLINTNEXTLINE(readability-convert-member-functions-to-static)