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: I534f5358e7c801ad5ad2c28185ab53b683ee3097
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index 6006b8c..261191b 100644
--- a/meson.build
+++ b/meson.build
@@ -1,7 +1,7 @@
 project(
     'openpower-debug-collector',
     'cpp',
-    meson_version: '>= 0.57.0',
+    meson_version: '>= 0.58.0',
     default_options: [
         'warning_level=3',
         'werror=true',