* guix/build/gnu-build-system.scm (set-paths, build, check, install):
New procedures.
(unpack): Make `source' a keyword arg; add `#:allow-other-keys'.
(configure): Likewise.
(%standard-phases): New variable.
(gnu-build): Make `source', `outputs', and `inputs' keyword arguments;
add `phases' keyword argument; #:allow-other-keys; add rest arguments
`args'. Invoke each of PHASES in order within `every'.
* guix/gnu-build-system.scm (gnu-build): Add `make-flags' and `phases'
keyword arguments. Update builder's `gnu-build' call to match the new
convention.
* guix/derivations.scm (build-expression->derivation): Add all of INPUTS
as inputs to the final derivation.
* guix/build/gnu-build-system.scm, guix/build/utils.scm,
guix/gnu-build-system.scm: New files.
* tests/builders.scm ("gnu-build"): New test.
* guix/derivations.scm (imported-modules): New procedure.
(build-expression->derivation): New keyword argument `modules'.
Use `imported-modules' when MODULES is non-empty, and pass it with
`-L' to GUILE.
* guix/build/http.scm, guix/http.scm, tests/builders.scm: New files.
