Abstract build systems.

* Makefile.am (MODULES): Add `guix/build-system.scm' and
  `guix/build-system/gnu.scm'.  Remove `guix/gnu-build-system.scm'.

* guix/build-system.scm: New file.
* guix/gnu-build-system.scm: Rename to...
* guix/build-system/gnu.scm: ... this.
  (gnu-build-system): New variable.

* tests/builders.scm: Adjust `use-module' clauses.
  ("gnu-build-system"): New test.

build: Be cross-compilation-ready.

* Makefile.am (.scm.go): Pass `--target=$(host)'.

* configure.ac: Use `AC_CANONICAL_HOST'.

build: Provide the right include path when building modules.

* Makefile.am (.scm.go): Add missing `-L $(top_srcdir)'.

Build (guix) last.

* Makefile.am (MODULES): Move `guix.scm' last.

Add a build system.

* configure.ac, Makefile.am: New files.