* guix/build/utils.scm (substitute): Change to accept a `pattern+procs'
parameter. Iterate over it.
(substitute*): Adjust accordingly.
* distro/base.scm (guile-1.8): Adjust accordingly.
* guix/build/utils.scm (let-matches, substitute*): New macros.
* distro/base.scm (guile-1.8): Use `substitute*' instead of
`substitute'. Remove the #:modules argument.
* 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.