macos.nim

proc foo*(x:string):string =
  return x

main.nim

proc foo*(x:string):string

when defined(macosx):
  import ./macos
else:
  import ./win

output.txt

$ nim c main.nim 
Hint: used config file '/Users/matt/lib/Nim/config/nim.cfg' [Conf]
Hint: system [Processing]
Hint: main [Processing]
Hint: macos [Processing]
main.nim(1, 6) Error: implementation of 'main.foo(x: string)[declared in main.nim(1, 5)]' expected

win.nim

proc foo*(x:string):string =
  return x