- If we cast macro arguments to void, we can avoid unused errors that have
  forced lots of conditionally included code like this:

```c
  #if defined(PETSC_USE_LOG)
  PetscLogEvent event;
  #endif
```

- Having global counters like petsc_TotalFlops externed event
  if !PetscDefined(USE_LOG) helps to remove a lot of needless
  `#ifdef`s around code that can instead be placed in
  `if (PetscDefined(USE_LOG)) {}` blocks.