Fix builds on recent ghc versions on Linux

[A1kmm]

No description provided.

[hvr]

@A1kmm this seem unnecessary...? What concrete problem does this fix? I haven't noticed any issues with double-conversion and GHC >= 8.0; and I don't see how this would be dependent on the GHC version.

[A1kmm]

When installing with Stack (with lts-12.5 as resolver) on Debian Stretch, I consistently get the same error as in issue #20, but it works as is with my older system ghc install.

Unclear if it is better to make this conditional on ghc version or something else, but I think if we can't find anything better, ghc version is the best available option to make it work on both newer and older installs likely to be found in practice.

[hvr]

@A1kmm I've tried to reproduce this on Debian 9 with https://downloads.haskell.org/~debian/ and everything works just fine. In fact, your "fix" would break double-conversion for everyone else (at the very least) on Debian and Ubuntu; if we had CI set up for this repo your PR would make CI fail.

I'm afraid this sounds like yet another Stack bug instance which seemed to be the underlying cause for #20; I suggest you try using Cabal as well (here's a quick intro to cabal new-build in case you need it) and see if that fixes the issue for you as well as it did in #20.