Install mostly static executables on OS X when using Nix (#830)

These aren't fully static executables (they still have some
`/nix/store` references), but they at least compile Haskell dependencies
statically.  That means that they can be `nix-env --install`ed side-by-side
with other Haskell executables, which would otherwise conflict with an
error like this one:

```
error: packages '/nix/store/hrxnlwlsiw5jjjkq5v6ihcwb0shx4fga-dhall-1.20.1/lib/li
nks/libHSbasement-0.0.8-8QjArDsw3GWCcbHE5iqtz3-ghc8.4.3.dylib' and '/nix/store/d
2y5373anwf1q3h86ar3lljk11k1lq0h-dhall-json-1.2.6/lib/links/libHSbasement-0.0.8-8
QjArDsw3GWCcbHE5iqtz3-ghc8.4.3.dylib' have the same priority 5; use 'nix-env --s
et-flag priority NUMBER INSTALLED_PKGNAME' to change the priority of one of the
conflicting packages (0 being the highest priority)
```

dhall-bash/default.nix

@@ -1 +1 @@
-(import ../nix/shared.nix {}).dhall-bash
+(import ../nix/shared.nix {}).possibly-static.dhall-bash

dhall-json/default.nix

@@ -1 +1 @@
-(import ../nix/shared.nix {}).dhall-json
+(import ../nix/shared.nix {}).possibly-static.dhall-json

dhall-text/default.nix

@@ -1 +1 @@
-(import ../nix/shared.nix {}).dhall-text
+(import ../nix/shared.nix {}).possibly-static.dhall-text

dhall/default.nix

@@ -1 +1 @@
-(import ../nix/shared.nix {}).dhall
+(import ../nix/shared.nix {}).possibly-static.dhall

nix/shared.nix

@@ -505,7 +505,7 @@ let
     then
       pkgsStaticLinux.pkgsMusl.haskell.packages."${compiler}"."${name}-static"
     else
-      pkgs.haskell.packages."${compiler}"."${name}";
+      pkgs.haskell.lib.justStaticExecutables (pkgs.haskell.packages."${compiler}"."${name}");
 
   makeTarball = name:
     pkgsStaticLinux.releaseTools.binaryTarball rec {