Only use jack as default driver on linux if it is installed

source/frontend/carla_shared.py

@@ -292,16 +292,18 @@ if WINDOWS:
     CARLA_DEFAULT_AUDIO_DRIVER = "DirectSound"
 elif MACOS:
     CARLA_DEFAULT_AUDIO_DRIVER = "CoreAudio"
-else:
+elif os.path.exists("/usr/bin/jackd") or os.path.exists("/usr/bin/jackdbus"):
     CARLA_DEFAULT_AUDIO_DRIVER = "JACK"
+else:
+    CARLA_DEFAULT_AUDIO_DRIVER = "PulseAudio"
 
-if LINUX:
+if CARLA_DEFAULT_AUDIO_DRIVER == "JACK":
     CARLA_DEFAULT_PROCESS_MODE   = ENGINE_PROCESS_MODE_MULTIPLE_CLIENTS
     CARLA_DEFAULT_TRANSPORT_MODE = ENGINE_TRANSPORT_MODE_JACK
 else:
     CARLA_DEFAULT_PROCESS_MODE   = ENGINE_PROCESS_MODE_PATCHBAY
     CARLA_DEFAULT_TRANSPORT_MODE = ENGINE_TRANSPORT_MODE_INTERNAL
-
+    
 # OSC
 CARLA_DEFAULT_OSC_ENABLED = not WINDOWS
 CARLA_DEFAULT_OSC_TCP_PORT_ENABLED = True