Add haskell.lib.setBuiltTarget, and support non library compiling of a single target

pkgs/test/haskell/setBuildTarget/default.nix

@@ -0,0 +1,26 @@
+{ pkgs, haskellPackages }: with pkgs.haskell.lib;
+
+
+let
+  drv     = haskellPackages.callCabal2nix "haskell-setBuildTarget" ./. {};
+  test    = target: excluded:
+
+    let only = setBuildTarget drv target;
+    in ''
+         if [[ ! -f "${only}/bin/${target}" ]]; then
+           echo "${target} was not built"
+           exit 1
+         fi
+
+         if [[ -f "${only}/bin/${excluded}" ]]; then
+           echo "${excluded} was built, when it should not have been"
+           exit 1
+         fi
+     '';
+
+in pkgs.runCommand "test haskell.lib.setBuildTarget" {} ''
+  ${test "foo" "bar"}
+  ${test "bar" "foo"}
+  touch "$out"
+''
+