Compiling with clang 3.8.1-18 (debian, x86_64) I ran across this
error:

gio-tool.c:40:31: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  message = g_strdup_vprintf (format, args);
                              ^~~~~~
gio-tool.c:55:31: error: format string is not a string literal [-Werror,-Wformat-nonliteral]
  message = g_strdup_vprintf (format, args);
                              ^~~~~~
2 errors generated.

To fix the first one, related with the function print_error(), this
patch adds to the function prototype a compiler's attribute.

For the second one, since the usage of that function is to print
one string and the format is already provided, the patch simplifies
the function by no receiving variadic arguments.

https://bugzilla.gnome.org/show_bug.cgi?id=781125