Undefined reference

[buster]

I am trying to connect to Jack (Kubuntu 18.04, so it is Jack2) from a Qt app:

void JackPortInfo::signal_handler ( int sig )
{
    jack_client_close ( client );
    qDebug() << "JackPortInfo: signal received, exiting ...";
    exit ( 0 );
}

void JackPortInfo::signal_handler ( int sig )
{
    jack_client_close ( client );
    qDebug() << "JackPortInfo: signal received, exiting ...";
    exit ( 0 );
}

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But I ran into a problem:
CMakeFiles/TestApp.dir/src/jackportinfo.cpp.o: In function `JackPortInfo::signal_handler(int)':
/home/me/projects/TestApp/src/jackportinfo.cpp:32: undefined reference to `jack_client_close'
collect2: error: ld returned 1 exit status

I thought
#include <jack/jack.h>
should resolve this, but it doesn't.

There is a bunch of jack related .h files in
/usr/src/linux-headers-4.15.0-74-generic/include/config/snd/
so it seems that what should be there is present.
Maybe I ought to post this question in a different forum?