dtrace support for FreeBSD

[dch]

h2o relies on picotls for dtrace detection in a CMake macro
and #241 mentions a possible
object relinking step that might make this possible. Can you
suggest anything to help implement this for picotls & h2o
on FreeBSD?

[kazuho]

Thank you for looking into the possibility.

IIRC, as stated on that issue, we need to pre-link the object files. MySQL used to do that (they recently dropped dtrace support, but...), and the code can be found at places like https://github.com/twitter-forks/mysql/blob/master/cmake/dtrace.cmake#L103-L119. I vaguely recall that it can be done slightly simpler, but anyways I think checking what MySQL does (did) would be a good starting point.