I think we shouldn't use it
http://answers.yahoo.com/question/index?qid=20071013052534AAuxWmW

and sorry for this link (italian)
http://forum.html.it/forum/showthread/t-422892.html

but to be sure we can use something like this
 #define fflush(stdin) while ((getchar()) != '\n')

would be better, right?