This is a first step towards bootstrapping from a set of pre-built,
statically-linked binaries.
* guix/build-system/gnu.scm (package-with-explicit-inputs,
standard-inputs): New procedure.
(%store): New variable.
(%standard-inputs): Remove.
(gnu-build): New `implicit-inputs?' keyword parameter. Use it to
choose whether to use `(standard-inputs SYSTEM)' or the empty list.
* distro/base.scm (guile-2.0): Remove dependency on XZ, which is now
implicit.
(%bootstrap-inputs, gcc-boot0, binutils-boot0, linux-headers-boot0,
%boot1-inputs, glibc-final, %boot2-inputs, m4-boot2, gmp-boot2,
mpfr-boot2, mpc-boot2, %boot3-inputs, gcc-final, %boot4-inputs,
%final-inputs): New variables.