Be more explicit; eliminates type error.  This'll go away when itpp goes
away.
add /opt/local -- helps for macports/homebrew
- use the polyphase resampler instead
- higher samp_rate (although 500k is fine on my N210)
1) Updates for GR 3.7.0git.  This should compile against both recent 3.6
(tested against 3.6.4.2) and current 3.7.

This makes things a bit ugly, as the main cmake file now both looks for
gnuradio-core (which is an indicator of 3.6) and tries the new 3.7 way
of locating GR.  The results are chosen and passed on to other cmake
files, and a new macro (GR_OLD) should be defined iff 3.6 is detected.

2) In pocencode_impl.cc, return the number of bytes actually
transferred fro work().
ignore the generated files
Initial hackrf support; this isn't working reliably for me yet.
Requires GR 3.7.0 + gr-osmosdr.
remove unnecessary debugging printf from work()