build: build with C23 if supported

This lets us conditionally leverage C23 features.

Force one build to C11 in CI to make sure we stay C11-compatible.

.builds/archlinux.yml

@@ -27,7 +27,7 @@ tasks:
   - setup: |
       cd wlroots
       CC=gcc meson setup build-gcc --fatal-meson-warnings --default-library=both -Dauto_features=enabled --prefix /usr -Db_sanitize=address,undefined
-      CC=clang meson setup build-clang --fatal-meson-warnings -Dauto_features=enabled
+      CC=clang meson setup build-clang --fatal-meson-warnings -Dauto_features=enabled -Dc_std=c11
   - gcc: |
       cd wlroots/build-gcc
       ninja

meson.build

@@ -5,7 +5,7 @@ project(
 	license: 'MIT',
 	meson_version: '>=0.59.0',
 	default_options: [
-		'c_std=c11',
+		'c_std=' + (meson.version().version_compare('>=1.3.0') ? 'c23,c11' : 'c11'),
 		'warning_level=2',
 		'werror=true',
 	],