since we no longer support 32-bit Mac builds.

The old build-darwin-i386 directory name appeared in a shocking number of
files. Change CMake paths to use ${CMAKE_BINARY_DIR} -- or, when trying to
find the packages subdirectory, ${AUTOBUILD_INSTALL_DIR}. Change the rest to
at least look for build-darwin-*.

This is the function in indra/llmessage/tests/testrunner.py that iterates
through ports in a specified range, looking for an available one. Other
platforms understand a specification of port 0 to mean: "You pick one. I'll
just use whichever one you picked."

But since the real problem is quite different, try with that suspected test
restored.

Instead of having testrunner.run()'s caller pass a Thread object on which to
run the caller's server instance's serve_forever() method, just pass the
server instance. testrunner.run() now constructs the Thread. This API change
allows run() to also call shutdown() on the server instance when done, and
then join() the Thread.

The hope is that this will avoid the Python runtime forcing the process
termination code to 1 due to forcibly killing the daemon thread still running
serve_forever().

While at it, eliminate calls to testrunner.freeport() -- just make the runtime
pick a suitable port instead.

Loath though I am to skip testing, this consistent failure is not a failure in
the software being tested (llcorehttp) but rather in the dummy server with
which we're testing it.

It's possible that raising an exception in a worker thread -- even though
we're TRYING to suppress it -- is what's causing the process to terminate with
nonzero rc.

This handles the case of building within an IDE, in which AUTOBUILD_ADDRSIZE
isn't already set.