Use C++11 thread_local instead of compiler extensions. (census-instrumentation#280)

Author: g-easy

opencensus/context/internal/context.cc

@@ -22,12 +22,6 @@
 #include "opencensus/tags/tag_map.h"
 #include "opencensus/trace/span.h"
 
-#if defined(_MSC_VER)
-#define TLS __declspec(thread)
-#else
-#define TLS __thread
-#endif
-
 namespace opencensus {
 namespace context {
 
@@ -54,7 +48,7 @@ std::string Context::DebugString() const {
 
 // static
 Context* Context::InternalMutableCurrent() {
-  static TLS Context* thread_ctx = nullptr;
+  static thread_local Context* thread_ctx = nullptr;
   if (thread_ctx == nullptr) thread_ctx = new Context;
   return thread_ctx;
 }