pipewire: handle --enable-pipewire correctly
With --enable-pipewire the variable enable_pipewire contains "yes", and is therefore set. Avoid the branch into AC_MSG_ERROR by checking have_pipewire, which indicates if libpipewire-0.3 exists.
Fixes commit f1ab721b ("pipewire: initial audio output module")
Signed-off-by: Olaf Hering olaf@aepfle.de