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: If15dcc5036a9d2a8f273a3baab659d2b521dec0d
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 77b10e4..2c473a2 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',
 )
 add_project_arguments('-Wno-psabi', language: 'cpp')
 
@@ -44,9 +44,9 @@
     sdbusplus = sdbusplus.as_system('system')
 endif
 systemd = dependency('systemd')
-systemd_system_unit_dir = systemd.get_pkgconfig_variable(
+systemd_system_unit_dir = systemd.get_variable(
     'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
+    pkgconfig_define: ['prefix', get_option('prefix')])
 packagedir = join_paths(
     get_option('prefix'),
     get_option('datadir'),
diff --git a/service_files/meson.build b/service_files/meson.build
index 5f7baf8..30d8377 100644
--- a/service_files/meson.build
+++ b/service_files/meson.build
@@ -15,9 +15,9 @@
     endif
 endforeach
 
-dbus_system_bus_services_dir = dependency('dbus-1').get_pkgconfig_variable(
+dbus_system_bus_services_dir = dependency('dbus-1').get_variable(
     'system_bus_services_dir',
-    define_variable: ['prefix', get_option('prefix')])
+    pkgconfig_define: ['prefix', get_option('prefix')])
 install_data(
     'dbus/xyz.openbmc_project.EntityManager.service',
     install_dir: dbus_system_bus_services_dir)