derivations: Add #:substitutable?, distinguished from #:local-build?.
Fixes <http://bugs.gnu.org/18747>.
* guix/derivations.scm (substitutable-derivation?): Rewrite to check for
"allowSubstitutes".
(derivation): Add #:substitutable? parameter.
[user+system-env-vars]: Honor it.
(build-expression->derivation): Add #:substitutable? and honor it.
* guix/gexp.scm (gexp->derivation): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and substitutes,
non-substitutable build"): Use #:substitutable? instead of #:local-build?.
("substitutable-derivation?", "derivation-prerequisites-to-build and
substitutes, local build"): New tests.
* guix/download.scm (url-fetch): Adjust comment.
* guix/git-download.scm (git-fetch): Likewise.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Use #:substitutable?
instead of #:local-build?.
* doc/guix.texi (Derivations, G-Expressions): Adjust accordingly.
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