mupen64plus: build with gcc49

Works around failure with gcc5, see
https://hydra.nixos.org/build/34273319/nixlog/1/raw
Not a real fix, but a working build is better than nothing ...

I have tested that the program at least runs: it fails to load a few
plugins, tho, have no idea to what extent that affects use
  Couldn't load plugin 'ricevideo.so':
  /nix/store/...-mupen64plus-1.5/share/mupen64plus/plugins/ricevideo.so:
  undefined symbol: glCombinerInputNV
  Couldn't load plugin 'glN64.so':
  /nix/store/...-mupen64plus-1.5/share/mupen64plus/plugins/glN64.so:
  undefined symbol: glCombinerInputNV