darwin: R: provide gettext and gfortran as buildInputs on Darwin.

As discussed in #10623, many R modules fail to build on Darwin without the
libraries and compilers provided by these packages.

For more detail, please see comment:

  https://github.com/NixOS/nixpkgs/pull/10623#issuecomment-172375342

pkgs/development/r-modules/default.nix

@@ -8,6 +8,7 @@ let
   buildRPackage = pkgs.callPackage ./generic-builder.nix {
     inherit R;
     inherit (pkgs.darwin.apple_sdk.frameworks) Cocoa Foundation;
+    inherit (pkgs) gettext gfortran;
   };
 
   # Generates package templates given per-repository settings