meson.build: fix detection of C23 auto

Sadly, this detection was broken because in C everything defaults to
type int. Casting a const char* to int is permitted but generates a
warning which was promptly ignored by meson.

This result in HAVE_C23_AUTO being set on compilers that don't by
default have -Werror=implicit-int and "auto" ended up being just an
"int".

Change the detection to use gmtime() which returns a struct, and add a
basic test using one of our struct-returning utility functions, just to
be sure.

Part-of: <https://gitlab.freedesktop.org/libinput/libinput/-/merge_requests/1198>

meson.build

@@ -134,7 +134,9 @@ if cc.has_header('xlocale.h')
 endif
 
 code = '''
-void func() { auto foo = "test"; }
+#include <time.h>
+
+void func() { auto foo = gmtime(NULL); foo->tm_sec = 0; }
 '''
 if cc.compiles(code, name: 'has C23 auto keyword')
 	config_h.set('HAVE_C23_AUTO', '1')

test/test-utils.c

@@ -51,6 +51,15 @@
 #define  TEST_VERSIONSORT
 #include "libinput-versionsort.h"
 
+START_TEST(auto_test)
+{
+	/* This one is just a compile test */
+	auto tv = us2tv(0);
+	tv.tv_sec = 0;
+	litest_assert_int_eq(tv.tv_sec, 0);
+}
+END_TEST
+
 START_TEST(mkdir_p_test)
 {
 	const char *testdir = "/tmp/litest_mkdir_test";
@@ -2258,6 +2267,7 @@ int main(void)
 	litest_runner_add_test(runner, &tdesc); \
 } while(0)
 
+	ADD_TEST(auto_test);
 	ADD_TEST(mkdir_p_test);
 
 	ADD_TEST(array_for_each);