ghc is buildable on darwin by hard-fixing darwin platform to 10.9.

ghc was not buildable on Mac OS  X darwin system (#1157), but with this workaround that fixes darwin platform to 10.9 in configure in a brute force way, ghc can be now buildable and works. 

Squashed commit of the following:

commit d8eb6cb00c8f20af01e9bbdfff5927d0ec85a749
Author: Ian-Woo Kim <ianwookim@gmail.com>
Date:   Wed Jan 15 01:05:00 2014 +0100

    remove xcodebuild back. working ghc without xcodebuild modification in stdenv

commit a22fa75e0683a150fb22b73137f73468d05de419
Author: Ian-Woo Kim <ianwookim@gmail.com>
Date:   Fri Jan 10 16:08:34 2014 +0100

    ghc-7.6.3 buildable on MacOSX darwin system with hard-coded platform information

commit f5a72128df12568f9751d87059c578dfedddab7c
Author: Ian-Woo Kim <ianwookim@gmail.com>
Date:   Thu Jan 9 22:27:20 2014 +0100

    gmp-4.3.2 is compilable with forced x86_64 architecture

commit 58f5dff22b7654a935b600b6b58d8065f938a18a
Author: Ian-Woo Kim <ianwookim@gmail.com>
Date:   Thu Jan 9 21:36:40 2014 +0100

    add xcodebuild in native-darwin-cctools-wrapper