meson: remove deprecated get_pkgconfig_variable

Since meson 0.56, the `get_pkgconfig_variable` has been deprecated.  In
meson 0.58 the `get_variable` was enhanced to no longer require the
`pkgconfig` keyword argument.  Ensure meson 0.58 is required and update
the usage of all `get_pkgconfig_variable` and `get_variable` to be the
modern variant.

Change-Id: Ic35c323182e32b7d780aa36228214094a416c45b
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/dbus_sensor_tester/meson.build b/dbus_sensor_tester/meson.build
index 31d28c5..fd51870 100644
--- a/dbus_sensor_tester/meson.build
+++ b/dbus_sensor_tester/meson.build
@@ -1,6 +1,6 @@
 project('dbus-sensor-tester', 'cpp',
         version : '1.0',
-        meson_version: '>=0.57.0',
+        meson_version: '>=0.58.0',
         default_options: [
             'b_lto_mode=default',
             'b_lto_threads=0',
@@ -124,7 +124,7 @@
 srcfiles_sensortest = [
 ]
 
-systemd_system_unit_dir = systemd.get_variable(pkgconfig: 'systemdsystemunitdir')
+systemd_system_unit_dir = systemd.get_variable('systemdsystemunitdir')
 bindir = get_option('prefix') + '/' +get_option('bindir')
 
 # Generate the executable