cabal sdist does not work with conditional flags in executables

[agocorona]

I have

 executable   monitorService
   if !impl(ghcjs >=0.1)
     main-is :      MonitorService.hs
     build-depends: base >4 && <5, transient, transient-universe, transformers
     default-language:  Haskell2010
     ghc-options:  -threaded -rtsopts
     hs-source-dirs: src/Transient/Move/Services
   else
     main-is : void.hs
     build-depends: base

cabal sdist
...
cabal.exe: MonitorService.hsvoid.hs doesn't exist

[23Skidoo]

Duplicate of #3313.