ghci is broken by `capi` calling convention

[vincenthz]

Related to #420

Compiling Foundation.System.Bindings.Posix ( .stack-work/dist/x86_64-linux-tinfo6/Cabal-2.0.0.2/build/Foundation/System/Bindings/Posix.hs, interpreted )
Error: bytecode compiler can't handle some foreign calling conventions
  Workaround: use -fobject-code, or compile this module to .o separately.

Options:

1. remove capi in favor of ignoring the variadic function (since it's not used this way on most system)
2. jump through a C binding function
3. hide capi under an ifdef for system that are not BSD,Linux,OSX (using the real variadic api for system that could be using it, hence maximing portability)