util: add a wrapper for the C23 auto keyword

C23 auto is basically __auto_type so let's wrap it if our compiler
doesn't provide it (yet).

This lets us use `auto` as type specifier, e.g. compare
   enum libinput_config_status status = libinput_device_config_set(...)
   auto status = libinput_device_config_set(...)

Note that as of now meson will never detect this as it requires -std=c23
to be passed to the compiler. This flag is only supported by Clang 18
(released 2024) and we don't want to break things for older compilers
for what is a bit of a niche feature right now.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1181>

meson.build

@@ -133,6 +133,13 @@ if cc.has_header('xlocale.h')
 	config_h.set('HAVE_XLOCALE_H', '1')
 endif
 
+code = '''
+void func() { auto foo = "test"; }
+'''
+if cc.compiles(code, name: 'has C23 auto keyword')
+	config_h.set('HAVE_C23_AUTO', '1')
+endif
+
 code = '''
 #include <locale.h>
 void main(void) { newlocale(LC_NUMERIC_MASK, "C", (locale_t)0); }

src/util-macros.h

@@ -27,6 +27,10 @@
 
 #include "config.h"
 
+#ifndef HAVE_C23_AUTO
+#define auto __auto_type
+#endif
+
 #define ARRAY_LENGTH(a) (sizeof (a) / sizeof (a)[0])
 /**
  * Iterate through the array _arr, assigning the variable elem to each