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: I72f4c91c762fa66ce749a93a77216cb4307ee7ca
Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
diff --git a/meson.build b/meson.build
index b22bec4..f6ce959 100644
--- a/meson.build
+++ b/meson.build
@@ -1,6 +1,6 @@
 # See README.md for details.
 project('openpower-hw-diags', 'cpp',
-        version: '0.1', meson_version: '>=0.57.0',
+        version: '0.1', meson_version: '>=0.58.0',
         default_options: [
           'warning_level=3',
           'werror=true',