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: Idff7506a1f897993ba350fdc31ce5103c8a9e998
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index ef7e203..cc67826 100644
--- a/meson.build
+++ b/meson.build
@@ -8,7 +8,7 @@
         'werror=true',
     ],
     license: 'Apache-2.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     version: '1.0',
 )
 
@@ -112,7 +112,7 @@
 )
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-    pkgconfig : 'systemdsystemunitdir'
+    'systemdsystemunitdir'
 )
 
 conf = configuration_data()
@@ -137,7 +137,7 @@
 endforeach
 
 dbus_system_bus_services_dir = dependency('dbus-1').get_variable(
-    pkgconfig: 'system_bus_services_dir',
+    'system_bus_services_dir',
     pkgconfig_define: ['prefix', get_option('prefix')]
 )