Fixed windows os.nim issue that was recently introduced.

lib/pure/os.nim

@@ -509,7 +509,7 @@ proc expandFilename*(filename: string): string {.rtl, extern: "nos$1".} =
     var L = GetFullPathNameA(filename, 3072'i32, result, unused)
     if L <= 0'i32 or L >= 3072'i32: OSError()
     setLen(result, L)
-  when defined(macosx):
+  elif defined(macosx):
     # On Mac OS X 10.5, realpath does not allocate the buffer on its own
     var pathBuffer: cstring = newString(pathMax)
     var resultBuffer = realpath(filename, pathBuffer)