We check for the existence of timer_settime() with some M4 macros from
gnulib. These POSIX APIs are required for building the profiler.

Allow disabling the profiler with a configure switch. This keeps the API
(and its preconditions as much as possible) but makes it a no-op and
prints an informational message at runtime on attempted use. We also
leave out the sysprof writer code and skip the profiler tests if the
profiler is not being built.