meson: decouple session tracking mechanism from unit and sysusers

Session tracking build options are about whether logind or other APIs
are used in the polkit code, at compilation and linking time.

units and sysusers files have nothing to do with code changes or APIs,
they simply install config files, that can just be ignored if they are
not needed. Unconditionally require a systemd.pc pkg-config file and
install these config files. When not available a stub that points all
variables to /dev/null can be used, which works everywhere and for all
settings without needing custom options on each project and variable.

This also fixes another bug,  'systemdsystemunitdir' is specified as
an option the systemd_dep variable is not defined, but the sysusers.d
directory lookup uses it, causing a build failure:

dh_auto_configure -- \
	-Dexamples=false \
	-Dintrospection=true \
	-Dman=true \
	-Dsystemdsystemunitdir=/usr/lib/systemd/system \
	-Dtests=true \
	-Dgtk_doc=true -Dsession_tracking=libsystemd-login
	cd obj-x86_64-linux-gnu && DEB_PYTHON_INSTALL_LAYOUT=deb LC_ALL=C.UTF-8 meson setup .. --wrap-mode=nodownload --buildtype=plain --prefix=/usr --sysconfdir=/etc --localstatedir=/var --libdir=lib/x86_64-linux-gnu -Dpython.bytecompile=-1 -Dexamples=false -Dintrospection=true -Dman=true -Dsystemdsystemunitdir=/usr/lib/systemd/system -Dtests=true -Dgtk_doc=true -Dsession_tracking=libsystemd-login
The Meson build system
Version: 1.3.1
Source dir: /builds/bluca/polkit/debian/output/source_dir
Build dir: /builds/bluca/polkit/debian/output/source_dir/obj-x86_64-linux-gnu
Build type: native build
Project name: polkit
Project version: 124

<...>

Run-time dependency libsystemd found: YES 255
Checking for function "sd_uid_get_display" with dependency libsystemd: YES
Checking for function "sd_pidfd_get_session" with dependency libsystemd: YES
../meson.build:222:37: ERROR: Unknown variable "systemd_dep".

Follow-up for 24f1e0a

data/meson.build

@@ -30,23 +30,21 @@ if enable_pam
   )
 endif
 
-if session_tracking == 'logind'
-  configure_file(
-    input: 'polkit.service.in',
-    output: '@BASENAME@',
-    configuration: service_conf,
-    install: true,
-    install_dir: systemd_systemdsystemunitdir,
-  )
+configure_file(
+  input: 'polkit.service.in',
+  output: '@BASENAME@',
+  configuration: service_conf,
+  install: true,
+  install_dir: systemd_systemdsystemunitdir,
+)
 
-  configure_file(
-    input: 'polkit.conf.in',
-    output: '@BASENAME@',
-    configuration: service_conf,
-    install: true,
-    install_dir: systemd_sysusers_dir,
-  )
-endif
+configure_file(
+  input: 'polkit.conf.in',
+  output: '@BASENAME@',
+  configuration: service_conf,
+  install: true,
+  install_dir: systemd_sysusers_dir,
+)
 
 install_data(
   'policyconfig-1.dtd',

meson.build

@@ -199,16 +199,6 @@ enable_logind = (session_tracking != 'ConsoleKit')
 if enable_logind
   if session_tracking == 'logind'
     logind_dep = dependency('libsystemd', not_found_message: 'logind support requested but libsystemd library not found')
-
-    # systemd unit / service files
-    systemd_systemdsystemunitdir = get_option('systemdsystemunitdir')
-    if systemd_systemdsystemunitdir == ''
-      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')
-    endif
-
-    systemd_sysusers_dir = systemd_dep.get_pkgconfig_variable('sysusers_dir', default: '/usr/lib/sysusers.d')
   else
     logind_dep = dependency('libelogind', not_found_message: 'elogind support requested but libelogind library not found')
   endif
@@ -222,6 +212,16 @@ config_h.set('HAVE_LIBSYSTEMD', enable_logind)
 
 config_h.set('HAVE_PIDFD_OPEN', cc.get_define('SYS_pidfd_open', prefix: '#include <sys/syscall.h>') != '')
 
+# systemd unit / sysuser file installation directories
+systemd_dep = dependency('systemd', not_found_message: 'systemd.pc pkg-config file required but not found')
+systemd_systemdsystemunitdir = get_option('systemdsystemunitdir')
+if systemd_systemdsystemunitdir == ''
+  # 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')
+endif
+
+systemd_sysusers_dir = systemd_dep.get_pkgconfig_variable('sysusers_dir')
+
 # User for running polkitd
 polkitd_user = get_option('polkitd_user')
 config_h.set_quoted('POLKITD_USER', polkitd_user)