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: I45c1ed610893020cd9c6479f25c22dd53e153750
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 35e1271..5a078f8 100644
--- a/meson.build
+++ b/meson.build
@@ -7,7 +7,7 @@
         'werror=true',
     ],
     license: 'Apache-2.0',
-    meson_version: '>=0.57.0',
+    meson_version: '>=0.58.0',
     version: '1.0',
 )
 
@@ -49,7 +49,7 @@
 endif
 
 systemd_system_unit_dir = dependency('systemd').get_variable(
-          pkgconfig: 'systemdsystemunitdir')
+          'systemdsystemunitdir')
 
 install_subdir('service_files',
                install_dir : systemd_system_unit_dir,