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: Ibc0692c5cd05546e797c20973e1c1f172dcb8379
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/subprojects/ncsid/meson.build b/subprojects/ncsid/meson.build
index af6037a..a813ba4 100644
--- a/subprojects/ncsid/meson.build
+++ b/subprojects/ncsid/meson.build
@@ -16,7 +16,7 @@
   'gbmc-ncsid',
   ['cpp', 'c'],
   version: '0.1',
-  meson_version: '>=0.57.0',
+  meson_version: '>=0.58.0',
   default_options: [
     'warning_level=3',
     'werror=true',