Bug 1357897 - ensure __has_feature(thread_local) before using it on Mac; r=froydnj

Bug 1348419 enabled thread-local storage for Mac, but only Xcode 8+
supports that feature, which busted building with Xcode 7-.  This change
fixes that bustage by ensuring a Mac compiler supports the feature
before using it.

mfbt/ThreadLocal.h

@@ -31,7 +31,8 @@ typedef sig_atomic_t sig_safe_t;
 
 namespace detail {
 
-#if defined(HAVE_THREAD_TLS_KEYWORD) || defined(XP_WIN) || defined(XP_MACOSX)
+#if defined(HAVE_THREAD_TLS_KEYWORD) || defined(XP_WIN) || \
+    (defined(XP_MACOSX) && defined(__has_feature) && __has_feature(cxx_thread_local))
 #define MOZ_HAS_THREAD_LOCAL
 #endif
 
@@ -181,7 +182,7 @@ ThreadLocal<T>::set(const T aValue)
 }
 
 #ifdef MOZ_HAS_THREAD_LOCAL
-#if defined(XP_WIN) || defined(XP_MACOSX)
+#if defined(XP_WIN) || (defined(XP_MACOSX) && defined(__has_feature) && __has_feature(cxx_thread_local))
 #define MOZ_THREAD_LOCAL(TYPE) thread_local mozilla::detail::ThreadLocal<TYPE>
 #else
 #define MOZ_THREAD_LOCAL(TYPE) __thread mozilla::detail::ThreadLocal<TYPE>