Don't install systemd units unless using libsystemd session tracking.

Submitted upstream:

  https://gitlab.freedesktop.org/polkit/polkit/-/merge_requests/134

diff --git a/data/meson.build b/data/meson.build
--- a/data/meson.build
+++ b/data/meson.build
@@ -26,7 +26,7 @@ if enable_pam
   )
 endif
 
-if enable_logind
+if session_tracking == 'libsystemd-login'
   configure_file(
     input: 'polkit.service.in',
     output: '@BASENAME@',
diff --git a/meson.build b/meson.build
--- a/meson.build
+++ b/meson.build
@@ -199,7 +199,7 @@ if enable_logind
 
   # systemd unit / service files
   systemd_systemdsystemunitdir = get_option('systemdsystemunitdir')
-  if systemd_systemdsystemunitdir == ''
+  if systemd_systemdsystemunitdir == '' and session_tracking == 'libsystemd-login'
     systemd_dep = dependency('systemd', not_found_message: 'systemd required but not found, please provide a valid systemd user unit dir or disable it')
     # FIXME: systemd.pc file does not use variables with relative paths, so `define_variable` cannot be used
     systemd_systemdsystemunitdir = systemd_dep.get_pkgconfig_variable('systemdsystemunitdir')
refs</a><a class='active' href='/guix/log/etc/completion/zsh'>log</a><a href='/guix/tree/etc/completion/zsh?id=5d182499e81a9325c3405691ea1bda23101bb172'>tree</a><a href='/guix/commit/etc/completion/zsh?id=5d182499e81a9325c3405691ea1bda23101bb172'>commit</a><a href='/guix/diff/etc/completion/zsh?id=5d182499e81a9325c3405691ea1bda23101bb172'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/completion/zsh'>
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<div class='path'>path: <a href='/guix/log/?id=5d182499e81a9325c3405691ea1bda23101bb172'>root</a>/<a href='/guix/log/etc?id=5d182499e81a9325c3405691ea1bda23101bb172'>etc</a>/<a href='/guix/log/etc/completion?id=5d182499e81a9325c3405691ea1bda23101bb172'>completion</a>/<a href='/guix/log/etc/completion/zsh?id=5d182499e81a9325c3405691ea1bda23101bb172'>zsh</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/etc/completion/zsh?id=5d182499e81a9325c3405691ea1bda23101bb172&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-10-24 20:17:05 +0200'>2021-10-24</span></td><td><a href='/guix/commit/etc/completion/zsh?id=55f6d9e818512b4e3e4adacfa99deb201b6854be'>etc: completion: Filter gratuitous spaces from available packages.</a><span class='msg-avail'>...<span class='msg-tooltip'>Tested with:

  guix environment --{ad-hoc,pure} zsh guix bash-completion coreutils -- zsh
  % fpath=(~/guix/etc/completion/zsh $fpath)
  % autoload -U compinit; compinit

* etc/completion/zsh/_guix (_guix_list_available_packages): Remove spaces.

Co-authored-by: Tobias Geerinckx-Rice &lt;me@tobias.gr&gt;
</span></span></td><td>Liliana Marie Prikler</td></tr>
<tr><td><span title='2021-08-28 17:57:50 +0200'>2021-08-28</span></td><td><a href='/guix/commit/etc/completion/zsh?id=a8dd285d5a0670abf124a721e6ba94da045b24ba'>zsh completion: Add missing commands and arguments.</a><span class='msg-avail'>...<span class='msg-tooltip'>* etc/completion/zsh/_guix (_guix_install, _guix_remove, _guix_upgrade):
  New functions.
  (_guix_build, _guix_challenge, _guix_download, _guix_environment,
  _guix_packages, _guix_gc, _guix_hash, _guix_lint, _guix_package,
  _guix_publish, _guix_pull, _guix_refresh, _guix_size, _guix_system):
  Add some of the missing arguments.
  (_guix_cmds): Add missing commands.

Co-authored-by: Brice Waegeneire &lt;brice@waegenei.re&gt;
</span></span></td><td>Noah Evans</td></tr>
<tr><td><span title='2021-02-17 10:52:28 +0100'>2021-02-17</span></td><td><a href='/guix/commit/etc/completion/zsh?id=ee2a5da80a9bda25542c00a7a35a9ddddcbd58af'>scripts: system: Remove 'vm-image' command.</a><span class='msg-avail'>...<span class='msg-tooltip'>Remove the 'vm-image' command that has been superseded by the 'image'
command.

* gnu/system/vm.scm (system-qemu-image): Remove it.
* guix/scripts/system.scm (system-derivation-for-action): Mark 'vm-image'
command as deprecated and use the image API to produce the VM image.
(perform-action, show-help): Adapt accordingly.
* tests/guix-system.sh: Ditto.
* doc/guix.texi (Invoking guix system,
Running Guix in a VM): Ditto.
* etc/completion/fish/guix.fish: Ditto.
* etc/completion/zsh/_guix: Ditto.
</span></span></td><td>Mathieu Othacehe</td></tr>