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')
712c03'>refs</a><a class='active' href='/guix/log/tests/file-systems.scm'>log</a><a href='/guix/tree/tests/file-systems.scm?id=f861793141d04ba737438330a239165991712c03'>tree</a><a href='/guix/commit/tests/file-systems.scm?id=f861793141d04ba737438330a239165991712c03'>commit</a><a href='/guix/diff/tests/file-systems.scm?id=f861793141d04ba737438330a239165991712c03'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/tests/file-systems.scm'>
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<div class='path'>path: <a href='/guix/log/?id=f861793141d04ba737438330a239165991712c03'>root</a>/<a href='/guix/log/tests?id=f861793141d04ba737438330a239165991712c03'>tests</a>/<a href='/guix/log/tests/file-systems.scm?id=f861793141d04ba737438330a239165991712c03'>file-systems.scm</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/tests/file-systems.scm?id=f861793141d04ba737438330a239165991712c03&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-05-20 08:30:35 -0400'>2020-05-20</span></td><td><a href='/guix/commit/tests/file-systems.scm?id=b460ba7992a0b4af2ddb5927dcf062784539ef7b'>bootloader: grub: Allow booting from a Btrfs subvolume.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/bootloader/grub.scm (strip-mount-point): Remove procedure.
(normalize-file): Add procedure.
(grub-configuration-file): New BTRFS-SUBVOLUME-FILE-NAME parameter.  When
defined, prepend its value to the kernel and initrd file names, using the
NORMALIZE-FILE procedure.  Adjust the call to EYE-CANDY to pass the
BTRFS-SUBVOLUME-FILE-NAME argument.  Normalize the KEYMAP file as well.
(eye-candy): Add a BTRFS-SUBVOLUME-FILE-NAME parameter, and use it, along with
the NORMALIZE-FILE procedure, to normalize the FONT-FILE and IMAGE nested
variables.  Adjust doc.
* gnu/bootloader/depthcharge.scm (depthcharge-configuration-file): Adapt.
* gnu/bootloader/extlinux.scm (extlinux-configuration-file): Likewise.
* gnu/system/file-systems.scm (btrfs-subvolume?)
(btrfs-store-subvolume-file-name): New procedures.
* gnu/system.scm (operating-system-bootcfg): Specify the Btrfs
subvolume file name the store resides on to the
`operating-system-bootcfg' procedure, using the new
BTRFS-SUBVOLUME-FILE-NAME argument.
* doc/guix.texi (File Systems): Add a Btrfs subsection to document the use of
subvolumes.
* gnu/tests/install.scm (%btrfs-root-on-subvolume-os)
(%btrfs-root-on-subvolume-os-source)
(%btrfs-root-on-subvolume-installation-script)
(%test-btrfs-root-on-subvolume-os): New variables.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2020-05-20 08:30:34 -0400'>2020-05-20</span></td><td><a href='/guix/commit/tests/file-systems.scm?id=fa35fb58c84d1c1741e4e63c0b37074e35ed2a61'>file-systems: Add helpers for parsing the options string into an alist.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/system/file-systems.scm (file-system-options-&gt;alist)
(alist-&gt;file-system-options): New procedures.
* tests/file-systems.scm: New tests.
* doc/guix.texi (File Systems): Add note about the newly added procedures.
</span></span></td><td>Maxim Cournoyer</td></tr>