Devel.hs

main :: IO ()
main = putStrLn "dev"

foo.cabal

name:                foo
version:             0.1.0.0
build-type:          Simple
cabal-version:       >=1.10

flag dev
  manual: True
  default: False
  description: foo

executable foo
  if flag(dev)
    main-is:             Devel.hs
  else
    main-is:             Main.hs
  build-depends:       base
  default-language:    Haskell2010

Main.hs

main :: IO ()
main = putStrLn "not dev"

Setup.hs

import Distribution.Simple
main = defaultMain