Work around weird unused errors with Mingw (#6075)

Summary:
From the reset of the code, it looks this this maybe can be unconditionally given the attribute? But I couldn't test with MSVC so I defensively put under CPP.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6075

Differential Revision: D18723749

fbshipit-source-id: 45fc8732c28dd29aab1644225d68f3c6f39bd69b

port/win/io_win.cc

@@ -1025,7 +1025,12 @@ Status WinRandomRWFile::Close() {
 //////////////////////////////////////////////////////////////////////////
 /// WinMemoryMappedBufer
 WinMemoryMappedBuffer::~WinMemoryMappedBuffer() {
-  BOOL ret = FALSE;
+  BOOL ret
+#if defined(_MSC_VER)
+    = FALSE;
+#else
+    __attribute__((__unused__));
+#endif
   if (base_ != nullptr) {
     ret = ::UnmapViewOfFile(base_);
     assert(ret);

port/win/win_thread.cc

@@ -138,7 +138,12 @@ void WindowsThread::join() {
       "WaitForSingleObjectFailed: thread join");
   }
 
-  BOOL rc;
+  BOOL rc
+#if defined(_MSC_VER)
+    = FALSE;
+#else
+    __attribute__((__unused__));
+#endif
   rc = CloseHandle(reinterpret_cast<HANDLE>(data_->handle_));
   assert(rc != 0);
   data_->handle_ = 0;

util/status.cc

@@ -21,7 +21,12 @@ const char* Status::CopyState(const char* state) {
 #ifdef OS_WIN
   const size_t cch = std::strlen(state) + 1;  // +1 for the null terminator
   char* result = new char[cch];
-  errno_t ret;
+  errno_t ret
+#if defined(_MSC_VER)
+    ;
+#else
+    __attribute__((__unused__));
+#endif
   ret = strncpy_s(result, cch, state, cch - 1);
   result[cch - 1] = '\0';
   assert(ret == 0);