* config-daemon.ac: Do not define NIX_VERSION.

* configure.ac: Require Autoconf 2.69.  Use 'AS_HELP_STRING' instead of
'AC_HELP_STRING'.
* m4/guix.m4: Likewise.
* config-daemon.ac: Use 'AC_CONFIG_HEADERS' instead of the singular
variant.

* config-daemon.ac: Check for 'statx'.
* nix/libstore/gc.cc (LocalStore::removeUnusedLinks) [HAVE_STATX]: Use
'statx' instead of 'lstat'.

As of GNU libc 2.29, <sys/mount.h> declares all the constants and
functions we need, so there's no use in including <linux/fs.h> anymore.
This silences annoying warnings like this one:

  In file included from nix/libstore/local-store.cc:32:0:
  /gnu/store/…-linux-libre-headers-4.19.56/include/linux/fs.h:108:0: warning: "MS_RDONLY" redefined
   #define MS_RDONLY  1 /* Mount read-only */

  In file included from nix/libstore/local-store.cc:28:0:
  /gnu/store/…-glibc-2.29/include/sys/mount.h:36:0: note: this is the location of the previous definition
   #define MS_RDONLY MS_RDONLY

* config-daemon.ac: Remove check for <linux/fs.h>.
* nix/libstore/build.cc: Remove conditional inclusion of <linux/fs.h>.
* nix/libstore/local-store.cc: Remove "#if HAVE_LINUX_FS_H" and
inclusion of <linux/fs.h>.

The daemon had a mechanism that allows it to handle a list of
substituters and try them sequentially; this removes it.

* nix/scripts/substitute.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove.
* config-daemon.ac: Don't output 'nix/scripts/substitute'.
* nix/libstore/build.cc (SubstitutionGoal)[subs, sub, hasSubstitute]:
Remove.
[tryNext]: Make private.
(SubstitutionGoal::SubstitutionGoal, SubstitutionGoal::init): Remove now
unneeded initializers.
(SubstitutionGoal::tryNext): Adjust to assume a single substituter: call
'amDone' upfront when we couldn't find substitutes.
(SubstitutionGoal::tryToRun): Adjust to run 'guix substitute' via
'settings.guixProgram'.
(SubstitutionGoal::finished): Call 'amDone(ecFailed)' upon failure
instead of setting 'state' to 'tryNext'.
* nix/libstore/globals.hh (Settings)[substituters]: Remove.
* nix/libstore/local-store.cc (LocalStore::~LocalStore): Adjust to
handle a single substituter.
(LocalStore::startSubstituter): Remove 'path' parameter.  Adjust to
invoke 'settings.guixProgram'.  Don't refer to 'run.program', which no
longer exists.
(LocalStore::querySubstitutablePaths): Adjust for 'runningSubstituters'
being a singleton instead of a list.
(LocalStore::querySubstitutablePathInfos): Likewise, and remove
'substituter' parameter.
* nix/libstore/local-store.hh (RunningSubstituter)[program]: Remove.
(LocalStore)[runningSubstituters]: Remove.
[runningSubstituter]: New field.
[querySubstitutablePathInfos]: Remove 'substituter' parameter.
[startSubstituter]: Remove 'substituter' parameter.
* nix/nix-daemon/guix-daemon.cc (main): Remove references to
'settings.substituters'.
* nix/nix-daemon/nix-daemon.cc (performOp): Ignore the user's
"build-use-substitutes" value when 'settings.useSubstitutes' is false.

* nix/scripts/offload.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS) [BUILD_DAEMON_OFFLOAD]:
Remove 'scripts/offload'.
* config-daemon.ac: Don't output 'nix/scripts/offload'.
* build-aux/pre-inst-env.in: Don't set 'NIX_BUILD_HOOK'.
* nix/libstore/build.cc (HookInstance::HookInstance): Run 'guix
offload'.
(DerivationGoal::tryBuildHook): Remove reference to 'NIX_BUILD_HOOK'.
* nix/nix-daemon/guix-daemon.cc (main) [HAVE_DAEMON_OFFLOAD_HOOK]: Don't
set 'NIX_BUILD_HOOK'.
* nix/nix-daemon/nix-daemon.cc (performOp) [!HAVE_DAEMON_OFFLOAD_HOOK]:
Leave 'settings.useBuildHook' unchanged.

* nix/scripts/download.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove 'scripts/download'.
* config-daemon.ac: Don't output 'nix/scripts/download'.
* nix/libstore/builtins.cc (builtinDownload): Invoke 'guix
perform-download' directly.

* nix/scripts/authenticate.in: Remove.
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove
scripts/authenticate.
* config-daemon.ac: Don't output 'nix/scripts/authenticate'.
* nix/libstore/local-store.cc (runAuthenticationProgram): Run 'guix
authenticate'.

* nix/scripts/list-runtime-roots.in: Remove.
* guix/store/roots.scm (%proc-directory): New variable.
(proc-file-roots, proc-exe-roots, proc-cwd-roots)
(proc-fd-roots, proc-maps-roots, proc-environ-roots)
(referenced-files, canonicalize-store-item, busy-store-items): New
procedures, taken from 'list-runtime-roots.in'.
* nix/libstore/globals.hh (Settings)[guixProgram]: New field.
* nix/libstore/globals.cc (Settings::processEnvironment): Initialize
'guixProgram'.
* nix/libstore/gc.cc (addAdditionalRoots): Drop code related to
'NIX_ROOT_FINDER'.  Run "guix gc --list-busy".
* nix/local.mk (nodist_pkglibexec_SCRIPTS): Remove
'scripts/list-runtime-roots'.
* config-daemon.ac: Don't output nix/scripts/list-runtime-roots.
* build-aux/pre-inst-env.in: Don't set 'NIX_ROOT_FINDER'.
Set 'GUIX'.
* doc/guix.texi (Invoking guix gc): Document '--list-busy'.
* guix/scripts/gc.scm (show-help, %options): Add "--list-busy".
(guix-gc)[list-busy]: New procedure.
Handle the 'list-busy' action.

* config-daemon.ac: Replace "ci.guix.info" with "ci.guix.gnu.org".
* doc/guix.texi (SUBSTITUTE-SERVER): Likewise.
* etc/substitutes/ci.guix.gnu.org.pub: New file.
* Makefile.am (dist_pkgdata_DATA): Add it.
* guix/scripts/build.scm (%default-log-urls): Update.
* guix/scripts/substitute.scm (%default-substitute-urls): Likewise.
* guix/store.scm (%default-substitute-urls): Likewise.
* guix/self.scm (miscellaneous-files): Add "ci.guix.gnu.org".