This patch extends the tracing infrastructure to support LTTng UserSpace
Tracer. Using LTTng-UST, users would have access to a low overhead tracing 
done entirely from userspace.

Depending on the tracing configure option used (none, --with-dtrace or
--with-lttngust) macros will be expanded to probes and tracepoints of the
desired tracer if any. Only the needed instrumentation will be added to the
binary. This technique is used to different degrees by other projects like 
Qemu[1] and Node.js[2].

I attached a patch adding this feature. I tested the changes and the 
instrumentation on LTTng-UST and SystemTap on Ubuntu 16.04. I would 
appreciate if someone could test those changes on macOS and other
platforms supporting USDT probes.

[1]https://github.com/qemu/qemu/blob/master/configure#L4303
[2]https://github.com/nodejs/node/blob/master/configure#L811