Building generated code should not emit warnings

[legrostdg]

See eskimor/servant-purescript#18

[freylax]

One cause of the warnings could be that the generated purescript code is not compatible with the purescript compiler in use. I will work for supporting purescript 0.11, see the configuration_options branch in my fork.

[safareli]

I have fixed similar issue in this project (link to pr)