build: Fix out-of-source-tree builds.

* Makefile.am (distro/packages/bootstrap/x86_64-linux/guile-bootstrap-2.0.6.tar.xz,
  distro/packages/bootstrap/i686-linux/guile-bootstrap-2.0.6.tar.xz):
  Make the target's parent directory.
* pre-inst-env.in (DISTRO_BOOTSTRAP_PATH): Add the builddir-relative
  directory.

distro: Change $DISTRO_{PATCH,BOOTSTRAP}_DIRECTORY to search paths.

* distro.scm (not-colon): New variable.
  (%patch-directory): Rename to...
  (%patch-path): ... this.  Turn into a list.  Expect $DISTRO_PATCH_PATH
  to be a colon-separated search path.
  (%bootstrap-binaries-directory): Rename to...
  (%bootstrap-binaries-path): ... this.  Likewise.
  (search-patch, search-bootstrap-binary): Adjust accordingly.

* pre-inst-env.in: Change to use `DISTRO_PATCH_PATH' and
  `DISTRO_BOOTSTRAP_PATH'.

build: Move `distro/patches' to `distro/packages/patches'.

* distro/patches: Move directory to...
* distro/packages/patches: ... here.
* Makefile.am (dist_patch_DATA): Adjust accordingly.
* pre-inst-env.in (DISTRO_PATCH_DIRECTORY): Likewise.

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.

Add `pre-inst-env' script.

* pre-inst-env.in: New file.
* configure.ac: Add it to `AC_CONFIG_FILES' and `AC_CONFIG_COMMANDS'.