Fix USB requirements.

Author: Francois Best

.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_EXTRA_ARGS="--lib=external/midi-usb/src"
+        - PLATFORMIO_CI_SRC=examples/MidiUSB REQUIRES_USB=1
         - PLATFORMIO_CI_SRC=examples/RPN_NRPN
         - PLATFORMIO_CI_SRC=examples/SimpleSynth
 
@@ -74,7 +74,11 @@ script:
     # Build current example
     - |
         if [ ! "${BUILD_UNIT_TESTS}" ]; then
-            platformio ci --lib="." --board=uno --board="due" --board="zero" --board="leonardo" $PLATFORMIO_CI_EXTRA_ARGS
+            if [ "${REQUIRES_USB}"]; then
+                platformio ci --lib="." --lib=external/midi-usb --board="due" --board="zero" --board="leonardo"
+            else
+                platformio ci --lib="." --board=uno --board="due" --board="zero" --board="leonardo"
+            fi
         fi
 
 after_success: