Use the pre-ratification spelling "c++2a" of the C++20 standard so that
meson 0.56.2 on Debian 11 accepts the cpp_std value ("c++20" was only
added to meson's cpp_std choices in 0.57). gcc and clang treat -std=c++2a
and -std=c++20 identically, and g++ 10 on bullseye compiles the C++ KMIP
client fine. The libkmip cmake subproject needs no change: cmake itself
emits -std=c++2a for CMAKE_CXX_STANDARD 20 on gcc 10.
--- a/meson.build
+++ b/meson.build
@@ -139,7 +139,7 @@ shared_module('pg_tde',
     'src/pg_tde_guc.c',
     'src/smgr/pg_tde_smgr.c',
   ),
-  override_options: ['cpp_std=c++20'],
+  override_options: ['cpp_std=c++2a'],
   include_directories: incdirs,
   install: true,
   install_dir: pkglibdir,
@@ -168,7 +168,7 @@ pg_tde_frontend = static_library('pg_tde_frontend',
   ),
   c_args: ['-DFRONTEND'],
   cpp_args: ['-DFRONTEND'],
-  override_options: ['cpp_std=c++20'],
+  override_options: ['cpp_std=c++2a'],
   dependencies: [pgfeutils, pgcommon, pgport, curl, crypto, ssl, kmipclient, kmipcore],
   include_directories: incdirs,
 )
