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: I964819bc87a6883694ad12a981ad6984d825662b
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 52953cc..b40793f 100644
--- a/meson.build
+++ b/meson.build
@@ -9,7 +9,7 @@
     ],
     license: 'Apache-2.0',
     version: '1.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
 )
 
 build_tests = get_option('tests')
@@ -27,9 +27,9 @@
               '-DBOOST_ERROR_CODE_HEADER_ONLY',
               '-DBOOST_SYSTEM_NO_DEPRECATED']
 
-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')])
 
 configure_file(
     copy: true,
@@ -57,9 +57,9 @@
 
 udev = dependency('udev')
 udev_rules_dir = join_paths(
-    udev.get_pkgconfig_variable(
+    udev.get_variable(
         'udevdir',
-        define_variable: ['prefix', get_option('prefix')],
+        pkgconfig_define: ['prefix', get_option('prefix')],
     ),
     'rules.d',
 )