This changeset adds a compile-time option on whether to include the
timing code.  If SPINS is compiled without TIMINGS=true, the timing code
is #defined to a no-op and should not otherwise affect the performance
of the software; timing_stack_report will instead print a short error
message.

Because this affects #defines in header files, changing from
TIMINGS=true to false (or vice versa) should only be done with a 'make
clean'.  Not doing so could leave the timing code in an inconsistent
state and result in either link-time errors (if older code calls timing
code that is #defined away) or a corrupted/growing timing stack (if for
example a timing_push() is compiled in but the corresponding
timing_pop() is removed.)

This commit adds stack-structured timing code, such that bits of SPINS
can be instrumented with

>  timing_push("short name")
>  [... work goes here]
>  timing_pop()

This is recursive, such that already-timed portions of the code can call
the timing code to further instrument subroutines or discrete code
segments, and the resulting structure follows the -dynamic- call stack.
This does have the odd result that portions of the code that are re-used
(namely spectral derivatives) can show up as several 'leaf' nodes in the
graph.

To print out the report to standard error, call

>  timing_stack_report()

The exact format of the report should be considered tentative.