grc: allow function probe with empty Block ID
cmake: fix FindPortaudio pkg-config support

Look for portaudio-2.0 which is the standard pkg-config name for the
portaudio API that gnuradio requires.  Correct the variable name in the
include directory hint derived from pkg-config information.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
cmake: fix FindPortaudio to reject unsupported API version

portaudio18 passes the generic test for locating portaudio, but will
fail when building gr-audio.  Test for the failure before accepting the
portaudio that was found.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>
filter: fix broken backported QA test in channelizer
utils: Added try: clause for SWIG import, fixing python-only OOT modules

__init__.py used to throw an ImportError when trying to load
a python-only OOT module.