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: I2e9338fdf0a00cbaa8293b03dabb25db78029392
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index cc27fbd..de8f9f2 100644
--- a/meson.build
+++ b/meson.build
@@ -8,7 +8,7 @@
     ],
     license: 'Apache-2.0',
     version: '1.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 fmt = dependency('fmt')
@@ -94,12 +94,12 @@
     ],
     install: true)
 
-systemd_system_unit_dir = dependency('systemd').get_pkgconfig_variable(
+systemd_system_unit_dir = dependency('systemd').get_variable(
     'systemdsystemunitdir',
-    define_variable: ['prefix', get_option('prefix')])
-udev_dir = dependency('udev').get_pkgconfig_variable(
+    pkgconfig_define: ['prefix', get_option('prefix')])
+udev_dir = dependency('udev').get_variable(
     'udev_dir',
-    define_variable: ['prefix', get_option('prefix')])
+    pkgconfig_define: ['prefix', get_option('prefix')])
 
 install_data(
     'xyz.openbmc_project.Hwmon@.service',