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: Ifcccb5c41390dea284834ace0ec286b019a995d3
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index fdbecfd..6c4b8ca 100644
--- a/meson.build
+++ b/meson.build
@@ -8,7 +8,7 @@
     ],
     license: 'Apache-2.0',
     version: '0.1',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 # Note, there is currently an issue with CPUSensor when used in conjunction
@@ -62,7 +62,7 @@
 
 systemd = dependency('systemd')
 systemd_system_unit_dir = systemd.get_variable(
-    pkgconfig: 'systemdsystemunitdir',
+    'systemdsystemunitdir',
     pkgconfig_define: ['prefix', get_option('prefix')])
 threads = dependency('threads')