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/attn/meson.build b/attn/meson.build
index 34ec50a..5a2834b 100644
--- a/attn/meson.build
+++ b/attn/meson.build
@@ -26,7 +26,7 @@
     output: 'attn_handler.service',
     install: true,
     install_dir:
-        dependency('systemd').get_variable(pkgconfig: 'systemdsystemunitdir')
+        dependency('systemd').get_variable('systemdsystemunitdir')
 )
 
 # install attention handler config file
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',