Don't keep both times and getrusage as backend: both are guaranteed by
gnulib, a single one suffices. Using getrusage is open to possibly
tracking other types of resources in the future.
* modules/timevar (Depends-on): Add getrusage.
(configure.ac): Remove gl_TIMEVAR.
(Files): Remove m4/timevar.m4.
* m4/timevar.m4: Remove, rely on gnulib for getrusage.
* lib/timevar.h (timevar_enabled): Clarify documentation.
* lib/timevar.c: Remove all the code about times.
Remove all the CPP guards about getrusage: expect it to be present
(courtesy of gnulib).