Omnithread library in trunk and release 3.2

The omnithread library is a C++ threading library that GNU Radio uses
for many of its internals. It comes from the omniORB CORBA library
and has been stripped down and included into the GNU Radio source
distribution since the 2.x days. We also make its services externally
available via libgromnithread.

We are transitioning away from using this library, and have already
begun to start using the Boost C++ threading library for things like
the thread-per-block scheduler.

The release 3.2 series will continue to have and make omnithread
available externally to maintain compatibility with users’ existing
code. However, eventually, we will be removing omnithread from the
trunk software, and will discontinue making it a part of GNU Radio in
the release 3.3 series.