Only use thread_local in builds with thread safety enabled

MSVC cannot dllexport thread_local variables, so we avoid making
globals thread local if we won't support multiple threads anyway.

Closes #3044

src/catch2/internal/catch_message_info.cpp

@@ -21,6 +21,6 @@ namespace Catch {
 
     // Messages are owned by their individual threads, so the counter should be thread-local as well.
     // Alternative consideration: atomic, so threads don't share IDs and things are easier to debug.
-    thread_local unsigned int MessageInfo::globalCount = 0;
+    CATCH_INTERNAL_THREAD_LOCAL unsigned int MessageInfo::globalCount = 0;
 
 } // end namespace Catch