$ dpkg -l | grep fldigi
ii fldigi 2.0-1
digital modem program for hamradio operators
a helper on linuxhamradio explained it this way:
"This is a bug in the Debian package, then. It should also depend on
Portaudio 19 is packaged a little unconventionally. The C and C++ headers
are both in portaudio19-dev, but the libs are in two different packages:
libportaudio2 and libportaudiocpp0. You are missing the latter."
I looked and port19-dev is not installed.
$ apt-get install libportaudio0
and now fldigi starts ok