Replace most gcc format attributes by macro GCC_FMT_ATTR (format checking)

Since version 4.4.x, gcc supports additional format attributes.
    __attribute__ ((format (gnu_printf, 1, 2)))
should be used instead of
    __attribute__ ((format (printf, 1, 2))
because QEMU always uses standard format strings (even with mingw32).

The patch replaces format attribute printf / __printf__ by macro
GCC_FMT_ATTR which uses gnu_printf if supported.

It also removes an #ifdef __GNUC__ (not needed any longer).

Cc: Blue Swirl <blauwirbel@gmail.com>
Signed-off-by: Stefan Weil <weil@mail.berlios.de>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>
diff --git a/monitor.h b/monitor.h
index 38b22a4..185cc3e 100644
--- a/monitor.h
+++ b/monitor.h
@@ -50,8 +50,7 @@
 int monitor_get_fd(Monitor *mon, const char *fdname);
 
 void monitor_vprintf(Monitor *mon, const char *fmt, va_list ap);
-void monitor_printf(Monitor *mon, const char *fmt, ...)
-    __attribute__ ((__format__ (__printf__, 2, 3)));
+void monitor_printf(Monitor *mon, const char *fmt, ...) GCC_FMT_ATTR(2, 3);
 void monitor_print_filename(Monitor *mon, const char *filename);
 void monitor_flush(Monitor *mon);