set default for llvm-ptx to False?

README says `This will use optimised backend implementations where available.` This sounds as if availability is checked in some automated way. Does not seem to work, as the `llvm-ptx` flag is True by default, and consequently (?) `cabal install` breaks if I don't have CUDA. Yes, I was able to figure out
```
cabal install accelerate-fft --flags="-llvm-ptx"
```
but you might make this flag default to False. - I guess `llvm-cpu: True` is fine since most people will have this installed.