distro: Add libsigsegv and GNU Awk.

* guix/packages.scm (<package>)[arguments]: Default to '().

* distro/base.scm (libsigsegv, gawk): New packages.
  (hello): Use GAWK instead of (nixpkgs-derivation "gawk").

packages: Add a `home-page' field.

* guix/packages.scm (<package>)[home-page]: New field.

* distro/base.scm (hello)[home-page]: Initialize.

Add a declarative packaging layer.

* Makefile.am (MODULES): Add `guix/packages.scm' and `distro/base.scm'.
  (TESTS): Add `tests/packages.scm'.
  (EXTRA_DIST): New variable.

* guix/packages.scm, distro/base.scm, tests/packages.scm: New files.

* guix/http.scm (http-fetch): Make `name' an optional argument, to match
  the expectations of `package-source-derivation'.