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: I3f360e25af1da4b026e70cce65480641ebcac263
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 59b5a38..9a0d49a 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',
 )
 
 build_tests = get_option('tests')
@@ -41,7 +41,7 @@
   output: 'lpcsnoop.service',
   configuration: conf_data,
   install: true,
-  install_dir: systemd.get_pkgconfig_variable('systemdsystemunitdir'))
+  install_dir: systemd.get_variable('systemdsystemunitdir'))
 
 executable(
   'snoopd',
@@ -67,13 +67,13 @@
 )
 
 if not get_option('7seg').disabled()
-  udevdir = dependency('udev', required : false).get_pkgconfig_variable('udevdir')
+  udevdir = dependency('udev', required : false).get_variable('udevdir')
   assert(udevdir != '', 'Cannot find udevdir')
   install_data(['80-7seg.rules'], install_dir : udevdir)
 
   install_data(
     ['postcode-7seg@.service'],
-    install_dir: systemd.get_pkgconfig_variable('systemdsystemunitdir')
+    install_dir: systemd.get_variable('systemdsystemunitdir')
   )
 
   executable(