distro: Use our own pre-built Guile to bootstrap.
* distro/packages/base.scm (%bootstrap-guile): Build from a tarball
containing a pre-built Guile.
* distro.scm (%bootstrap-binaries-directory): New variable.
(search-bootstrap-binary): New procedure.
* Makefile.am (bootstrapdir, bootstrap_x86_64_linuxdir,
dist_bootstrap_x86_64_linux_DATA, DISTCLEANFILES, DOWNLOAD_FILE): New
variables.
(distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz):
New rule.
(EXTRA_DIST): Add `build-aux/download.scm'.
(.scm.go): Define the `DISTRO_BOOTSTRAP_DIRECTORY' and
`DISTRO_INSTALLED_BOOTSTRAP_DIRECTORY' environment variables.
* pre-inst-env.in: Define `DISTRO_BOOTSTRAP_DIRECTORY'.
* build-aux/download.scm: New file.
* distro/packages/bootstrap/x86_64-linux/{bash, mkdir, tar, xz}: New
files.
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