Add basic "nix build" command

Currently only builds by attribute from <nixpkgs> or the specified
file, e.g. "nix build hello".

src/nix/installables.hh

@@ -0,0 +1,38 @@
+#pragma once
+
+#include "args.hh"
+
+namespace nix {
+
+struct UserEnvElem
+{
+    Strings attrPath;
+
+    // FIXME: should use boost::variant or so.
+    bool isDrv;
+
+    // Derivation case:
+    Path drvPath;
+    StringSet outputNames;
+
+    // Non-derivation case:
+    PathSet outPaths;
+};
+
+typedef std::vector<UserEnvElem> UserEnvElems;
+
+struct MixInstallables : virtual Args
+{
+    Strings installables;
+    Path file = "<nixpkgs>";
+
+    MixInstallables()
+    {
+        mkFlag('f', "file", "file", "evaluate FILE rather than the default", &file);
+        expectArgs("installables", &installables);
+    }
+
+    UserEnvElems evalInstallables(ref<Store> store);
+};
+
+}