Libusrp2 no longer needs omnithreads

The latest git master branch has an updated libusrp2 that no longer
requires the use of the omnithreads library. Instead, all functions
previously handled by omnithreads are handled by the boost threading

This has been a transition in progress for some time, so it doesn’t
change any compilation requirements or prerequisites for GNU Radio

For those who may be building libusrp2 only, you may now use the the

$ ./configure --disable-all-components --enable-gruel --enable-usrp2

…to configure the GNU Radio source code tree.

The omnithreads library has been deprecated and we’ve been slowly
migrating functionality to use the boost threading library (via the
gruel component.) At this point, there are only these remaining
components in the tree which depending on omnithreads:

gcell (mutexes and condition variables only)
gr-audio-osx (via the mld_threads class)
usrp (via the mld_threads class)

The gcell component can be changed easily, but will need testers on
that platform to verify. The mblock library is deprecated and will
likely not be converted. The last two components need some work to
convert to Boost threads, but I haven’t looked into this yet
(Michael–can you address this?).