Refactor `allGhcOptions` helper
Refactor `allGhcOptions` helper

[Data.Map.findWithDefault](http://hackage.haskell.org/package/containers-0.5.6.3/docs/Data-Map-Strict.html#v:findWithDefault) to the rescue.
MichielDerhaeg/stack,Heather/stack,Heather/stack,sjakobi/stack,keent/stack,AndrewRademacher/stack,narrative/stack,mrkkrp/stack,harendra-kumar/stack,Blaisorblade/stack,AndrewRademacher/stack,mrkkrp/stack,juhp/stack,narrative/stack,rubik/stack,anton-dessiatov/stack,harendra-kumar/stack,rvion/stack,achernyak/stack,Fuuzetsu/stack,MichielDerhaeg/stack,keent/stack,Blaisorblade/stack,AndreasPK/stack,phadej/stack,martin-kolinek/stack,Fuuzetsu/stack,phadej/stack,deech/stack,martin-kolinek/stack,rvion/stack,achernyak/stack,luigy/stack,sjakobi/stack,luigy/stack,deech/stack,vigoo/stack,AndreasPK/stack,anton-dessiatov/stack,rubik/stack,vigoo/stack,juhp/stack