[Patch-gnuradio] Building gnuradio - system libraries and gnuradio provided libraries

[A. Maitland Bottoms]

So during today's conference call the case came up of
the build picking up system installed libraries instead of
the libraries built during a build of gnuradio with source
distributed with gnuradio source.

This needs to be carefully configurable...

I'd been thinking about how to migrate volk out of
the gnuradio source, since it could be useful in other
applications. But since gnuradio and volk are so symbiotic
now that is a bad example.

Instead...

Consider the gsm codec distributed in gnuradio, from
Jutta Degener and Carsten Bormann. Debian already has
a libgsm source package
http://packages.debian.org/source/squeeze/libgsm
and I'd like the Debian packages to build-depend upon
libgsm1-dev and not use the source shipped with gnuradio.

Security maintainers really dislike finding a bug in a source
file that shows up copied in multiple places.

It's been great to have codec2 code used in gnuradio,  but
peering into the future - that too would be a library that
Debian would provide separately. Also now might be a good
time to think about how gnuradio might make use of the Opus
codec should that arrive on the scene soon. And it is past
time to think about using the speex codec. Since I haven't
gotten around to doing anything about incorporating these
codecs into grc blocks I'll at least think aloud and spur
discussion. Perhaps it is best to stage these blocks first
as cgran projects and then think about merging them into
gnuradio?

But my main request is that CMake selection of "convenience
copies" of source should be easy to configure to be 
overridden by system libraries when they are found.

The default should differ from library to library -
a system libgsm, if found, should be used instead of
the convenience copy gsm. Yet, the gnuradio source copy
of libgruel needs to be preferred over one found on
the system leftover from a previous gnuradio install.

And it should be easy and clear to force these choices
at configure time for a specific build.

Sorry for not sending code to patch-gnuradio - but I figure
I'd write down my ideas for you to read rather than expect
you just to read my mind.

-Maitland