c++ std and gnuradio - trouble and fixing

[Greg Troxel <gdt%lexort.com@localhost>]

Apparently recent gnuradio needs C++20, but the bl3 doesn't say that.

It's not clear if the headers need C++20 to be used.  I have found that
compiling gr-osmosdr with forcing c++11 (that gr-osmosdr is
cmake-documented to need) fails, and succeeds with forcing c++17.

Either way, 'import osmosdr' in python fails, unless gr-osmosdr is built
with gcc12.  On NetBSD 10, gcc10 is used - because it's good enough for
c++11, and because gnuradio's bl3 doesn't say.   I am not sure if the
failure is "you need to use C++20 to read gnuradio headers" or "gcc10
and gcc12 have incompatible ABIs" aka "c++ is a mess; film at 11" :-)

I have a query to upstream about gr-osmosdr, and will likely query about
headers c++ versions, after reading a bit more.

But I think we're heading to setting USE_LANGUAGES and USE_CXX_FEATURES
in bl3.