gdatetime: Enable compile time check of g_date_time_format() format

By annotating it with G_GNUC_STRFTIME.

https://gitlab.gnome.org/GNOME/glib/issues/1575

glib/gdatetime.h

@@ -261,7 +261,7 @@ GDateTime *             g_date_time_to_utc                              (GDateTi
 
 GLIB_AVAILABLE_IN_ALL
 gchar *                 g_date_time_format                              (GDateTime      *datetime,
-                                                                         const gchar    *format) G_GNUC_MALLOC;
+                                                                         const gchar    *format) G_GNUC_MALLOC G_GNUC_STRFTIME (2);
 
 G_END_DECLS
 

glib/tests/gdatetime.c

@@ -25,6 +25,14 @@
 #include <glib/gstdio.h>
 #include <locale.h>
 
+/* The tests below exercise invalid formats and y2k-unsafe formats,
+ * so we need to silence the warnings here to pass.
+ */
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wformat"
+#pragma GCC diagnostic ignored "-Wformat-y2k"
+#endif
+
 #define ASSERT_DATE(dt,y,m,d) G_STMT_START { \
   g_assert_nonnull ((dt)); \
   g_assert_cmpint ((y), ==, g_date_time_get_year ((dt))); \

tests/datetime.c

@@ -18,6 +18,14 @@
 #include <locale.h>
 #include <glib/glib.h>
 
+/* The whole purpose of this test is to pass the command line argument
+ * to g_date_time_format(), so the format argument cannot be compile-
+ * time checked. Disable the inevitable compiler warning.
+ */
+#ifdef __GNUC__
+#pragma GCC diagnostic ignored "-Wformat-nonliteral"
+#endif
+
 int
 main (int argc, char *argv[])
 {