build: modernize meson usage

Since we require meson 1.3(.2), we can modernize the usage a bit,
as well as improve some logic

- use / instead of join_paths
- use option.allowed() instead of !option.disable()
- use cxx.get_supported_arguments
- use fs.copyfile instead of configure_file
- show a summary at the end of the configuration

lib/message/meson.build

@@ -1,4 +1,4 @@
-## Copyright (C) 2022-2025 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
+## Copyright (C) 2022-2026 Dirk-Jan C. Binnema <djcb@djcbsoftware.nl>
 ##
 ## This program is free software; you can redistribute it and/or modify
 ## it under the terms of the GNU General Public License as published by
@@ -43,6 +43,6 @@ lib_mu_message_dep = declare_dependency(
   include_directories:
     include_directories(['.', '..']))
 
-if not get_option('tests').disabled()
+if get_option('tests').allowed()
   subdir('tests')
 endif