Enable Cabal flags llvm, gmp, and libffi in Idris.
Enable Cabal flags llvm, gmp, and libffi in Idris.

Cc: da39a3ee5e6b4b0d3255bfef95601890afd80709@shlevy

gridaphobe/cabal2nix,dmalikov/cabal2nix,psibi/cabal2nix,Fuuzetsu/cabal2nix,bennofs/cabal2nix,jb55/cabal2nix,spencerjanssen/cabal2nix