Sprinkle G_GNUC_PRINTF() to avoid the following warnings and get some new ones.

message.c:180:38: warning: format string is not a string literal [-Wformat-nonliteral]
  len = g_printf_string_upper_bound (fmt, args);
                                     ^~~
message.c:190:18: warning: format string is not a string literal [-Wformat-nonliteral]
  vsprintf (buf, fmt, args2);
                 ^~~
dialib.c:46:38: warning: format string is not a string literal [-Wformat-nonliteral]
  len = g_printf_string_upper_bound (fmt, args);
                                     ^~~
dialib.c:57:18: warning: format string is not a string literal [-Wformat-nonliteral]
  vsprintf (buf, fmt, args2);
                 ^~~
diacontext.c:162:27: warning: format string is not a string literal [-Wformat-nonliteral]
  msg = g_strdup_vprintf (format, args);
                          ^~~~~~
diacontext.c:181:27: warning: format string is not a string literal [-Wformat-nonliteral]
  msg = g_strdup_vprintf (format, args);
                          ^~~~~~