Only require C++14 for Windows builds

Summary: Because reasons :(

Reviewed By: yfeldblum

Differential Revision: D9923447

fbshipit-source-id: 7a3a9ab86764dc5c7f5b70f2f728aec8b67bdf1f

folly/portability/PThread.cpp

@@ -102,7 +102,7 @@ DWORD internal_pthread_thread_start(void* arg) {
   auto startupInfo = reinterpret_cast<pthread_startup_info*>(arg);
   current_thread_self = startupInfo->thread;
   auto ret = startupInfo->startupFunction(startupInfo->startupArgument);
-  if constexpr (sizeof(void*) != sizeof(DWORD)) {
+  if /* constexpr */ (sizeof(void*) != sizeof(DWORD)) {
     auto tmp = reinterpret_cast<uintptr_t>(ret);
     if (tmp > std::numeric_limits<DWORD>::max()) {
       throw std::out_of_range(