[Lmuse-user] GCC 7 fixes

[Tim <ter...@ro...>]

Hi just a heads up, I pushed a bunch o' fixes to master for
  MusE failing compile with the new gcc 7.

- LV2 GTK2 support is now optional, due to a current exception breakage 
in gtkmm.
- Fixed many places using switch/case 'fallthrough'. With gcc7 these now 
generate
    warnings which MusE treats as errors. I used 'comment warning 
suppressor indicators':
    // FALLTHROUGH
- Fixed slight de-referencing problem in JackAudioDevice::portName().

Tim.