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: If11ba12cb1999a11d4a2e3ad8c43cb0cfa1025fa
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 758f670..913918b 100644
--- a/meson.build
+++ b/meson.build
@@ -1,5 +1,5 @@
 project('ssifbridge', 'cpp',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     default_options: [
         'buildtype=debugoptimized',
         'warning_level=3',
@@ -21,9 +21,9 @@
 
 # Configure and install systemd unit files
 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')])
 
 configure_file(
     copy: true,