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: I7a0cd48c848ac3f50b2fd31bd43615f42a1e1807
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 3f69f8d..ab2c1f1 100644
--- a/meson.build
+++ b/meson.build
@@ -10,7 +10,7 @@
     ],
     license: 'Apache-2.0',
     version: '1.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 add_project_arguments('-Wno-psabi', language: 'cpp')
 
@@ -228,9 +228,9 @@
     'service_files/op-enter-mpreboot@.service',
 ] + extra_unit_files
 
-systemd_system_unit_dir = dependency('systemd').get_pkgconfig_variable(
+systemd_system_unit_dir = dependency('systemd').get_variable(
     'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
+    pkgconfig_define: ['prefix', get_option('prefix')])
 foreach u : unit_files
     configure_file(
         configuration: unit_subs,