* guix/build-system/gnu.scm (package-with-explicit-inputs): New `guile'
keyword parameter. Add it to P's arguments, and pass it in recursive
calls.
(gnu-build): New `guile' keyword parameter; new `guile-for-build'
variable. Pass it as the `#:guile-for-build' parameter of
`build-expression->derivation'.
* guix/build-system/trivial.scm (trivial-build): Likewise.
* guix/build-system/trivial.scm: New file.
* Makefile.am (MODULES): Add it.
* tests/packages.scm ("trivial"): New test.
* guix/packages.scm (package-derivation): Allow SOURCE to be #f.