dtrace

[Gerd Möllmann]

Please find attached the beginnings of DTrace support. The diff is
based on my Emacs but probably applies to scratch/igc as well.

This includes:

- detect dtrace at configure time. Default is to use it.
  Use --with-dtrace=no to disable.

- Building with DTrace support. This is only tested on macOS so far.
  Other OSs require compiling and linking in an additional .o. I think
  I've got that right, but cannot test it.

- One DEFUN in igc.c, igc-test-probe to produce a probe.

Not much, but I like it :-).

I'm now searching for a DTrace client to use this with. Haven't found a
nice one yet. I someone has used one and has a tip, please let me know!

If nothing else is there, one could probably build something with
Python/Jupyter, but I have to finish reading PyPI first :-).

dtrace.diff
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