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: I0aa20f956a896d11ba062f3df4a947f9e68dfb53
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/firstboot/meson.build b/firstboot/meson.build
index 06a139e..8b3dee7 100644
--- a/firstboot/meson.build
+++ b/firstboot/meson.build
@@ -13,7 +13,7 @@
         output: 'first-boot-set-mac@.service',
         copy: true,
         install_dir:
-            set_mac.get_pkgconfig_variable('systemdsystemunitdir'),
+            set_mac.get_variable('systemdsystemunitdir'),
     )
 endif
 
@@ -29,6 +29,6 @@
         output: 'first-boot-set-hostname.service',
         copy: true,
         install_dir:
-            set_hostname.get_pkgconfig_variable('systemdsystemunitdir'),
+            set_hostname.get_variable('systemdsystemunitdir'),
     )
 endif
diff --git a/http-redirect/meson.build b/http-redirect/meson.build
index 840261b..758b55e 100644
--- a/http-redirect/meson.build
+++ b/http-redirect/meson.build
@@ -19,7 +19,7 @@
             output: unit,
             copy: true,
             install_dir:
-                redirect.get_pkgconfig_variable('systemdsystemunitdir'),
+                redirect.get_variable('systemdsystemunitdir'),
         )
     endforeach
 endif