Omnithread is now a top-level component

FYI, r4328 extracts omnithread from gnuradio-core and makes it an
top-level component. This allows the mblock code (and others) to use
it without a dependency on gnuradio-core.

Packagers, please note: there is new library that gets installed in
PREFIX/lib, libgromnithread.so. The funny name is to avoid a conflict
with a Debian’s libomnithread.so

Linux users: after updating, building and installing, you’ll probably
need to do:

$ sudo ldconfig

to update the linker cache. (Only required if you’re installing into
a directory listed in /etc/ld.so.conf)

Eric