timevar: further work

[Bruno Haible]

Hi Akim,

- Unclear comments: timevar_enabled: When does it need to be set? Before or
  after init_timevar() is called?

- Portability: According to the Gnulib documentation, 'times' is missing on
  native Windows (mingw, MSVC). The Gnulib module 'times' has a replacement for
  it, that is better than the clock() fallback. How about making 'timevar'
  depend on the Gnulib module 'times'? Then you can ditch at least the
  USE_CLOCK code.

- Is the 'times' approach preferrable to the 'getrusage' approach in all
  cases? I'm thinking of multithreading and of processes that fork() children.

Bruno