update to C++20

Signed-off-by: Zane Shelley <zshelle@us.ibm.com>
Change-Id: I6958180551f4a8265be57661844a81d9441f086e
diff --git a/attn/meson.build b/attn/meson.build
index 8430c07..8c7d95f 100644
--- a/attn/meson.build
+++ b/attn/meson.build
@@ -22,8 +22,7 @@
     output: 'attn_handler.service',
     install: true,
     install_dir:
-        dependency('systemd').get_pkgconfig_variable(
-            'systemdsystemunitdir')
+        dependency('systemd').get_variable(pkgconfig: 'systemdsystemunitdir')
 )
 
 # see if phosphor-logging is available, if not use test case logging code
diff --git a/meson.build b/meson.build
index 75d56df..096b4bc 100644
--- a/meson.build
+++ b/meson.build
@@ -1,10 +1,10 @@
 # See README.md for details.
 project('openpower-hw-diags', 'cpp',
-        version: '0.1', meson_version: '>=0.51.0',
+        version: '0.1', meson_version: '>=0.57.0',
         default_options: [
           'warning_level=3',
           'werror=true',
-          'cpp_std=c++17',
+          'cpp_std=c++20',
         ])
 
 # Package directory root, which will contain required data files.