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/test/perf/low_memory/low-memory.cc

@@ -57,7 +57,7 @@ bool has_pressure()
     return false;
   }
 
-  uint64_t current_epoch = default_memory_provider.low_memory_epoch();
+  uint64_t current_epoch = default_memory_provider().low_memory_epoch();
   bool result = epoch != current_epoch;
   epoch = current_epoch;
   return result;