Add dep to MidiUSB for specific example & guards for non-usb boards.

.travis.yml

@@ -26,7 +26,7 @@ env:
         - PLATFORMIO_CI_SRC=examples/Callbacks
         - PLATFORMIO_CI_SRC=examples/DualMerger
         - PLATFORMIO_CI_SRC=examples/Input
-        - PLATFORMIO_CI_SRC=examples/MidiUSB
+        - PLATFORMIO_CI_SRC=examples/MidiUSB PLATFORMIO_CI_EXTRA_ARGS="--lib=external/midi-usb/src"
         - PLATFORMIO_CI_SRC=examples/RPN_NRPN
         - PLATFORMIO_CI_SRC=examples/SimpleSynth
 
@@ -74,7 +74,7 @@ script:
     # Build current example
     - |
         if [ ! "${BUILD_UNIT_TESTS}" ]; then
-            platformio ci --lib="." --board=uno --board="due" --board="zero" --board="leonardo"
+            platformio ci --lib="." --board=uno --board="due" --board="zero" --board="leonardo" $PLATFORMIO_CI_EXTRA_ARGS
         fi
 
 after_success:

examples/MidiUSB/MidiUSB.ino

@@ -1,4 +1,6 @@
 #include <MIDI.h>
+
+#if defined(USBCON)
 #include <midi_UsbTransport.h>
 
 static const unsigned sUsbTransportBufferSize = 16;
@@ -8,6 +10,10 @@ UsbTransport sUsbTransport;
 
 MIDI_CREATE_INSTANCE(UsbTransport, sUsbTransport, MIDI);
 
+#else // No USB available, fallback to Serial
+MIDI_CREATE_DEFAULT_INSTANCE();
+#endif
+
 // --
 
 void handleNoteOn(byte inChannel, byte inNumber, byte inVelocity)