Rename <package-source> to <origin>; use the `letrec*' behavior in packages.

* guix/packages.scm (<package-source>): Rename to...
  (<origin>): ... this.  Update users.

* distro/base.scm (libsigsegv, gawk, hello): Adjust to renaming; refer
  to VERSION to build the URL.

Add a `base32' macro, for convenience.

* guix/packages.scm (base32): New macro.

* distro/base.scm (libsigsegv, gawk, hello): Use it.

packages: Allow the `arguments' field to be a procedure.

* guix/packages.scm (package-derivation): When ARGS is a procedure, call
  (ARGS SYSTEM).

* distro/base.scm (gawk)[arguments]: Turn into a procedure, to disable
  tests on Cygwin.

distro: libsigsegv: Use multiple outputs.

* distro/base.scm (libsigsegv)[outputs]: New field
  (gawk)[arguments]: Clear.
  [inputs]: Add the "lib" output of LIBSIGSEGV.

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'.