Change the timing function.
Use a more accurate timer
Add C timings to profiling stats
Add VBW computation based on C timings
Add c time return for openmp backend.
Add _md5 property to kernel.
Properly time par loops over empty Sets
Use likwid header/library only if likwid profiling enabled
Guard likwid macros with ifdefs
Fix timing for lazily execute par loops

Since lazy par loops are initially only enqueued but executed later
when evaluation is enforced, we can't return the timing for the
par_loop call. The only meaningful place to measure timings is inside
the par_loop's _run method.
Include Cythonized likwid extension module in sdist
Only build likwid extension module if likwid is found

The check could be refined, it uses the hard-coded default likwid path.