Reduce dependence on C++ runtime

If the external thread statics are used, then
we don't need to include some C++ runtime
concepts. This refactoring moves some global initialization under
conditional compilation.

src/mem/pool.h

@@ -41,7 +41,7 @@ namespace snmalloc
 
     static Pool* make() noexcept
     {
-      return Pool::make(default_memory_provider);
+      return Pool::make(default_memory_provider());
     }
 
     template<typename... Args>