implement the nonstandard GNU function fpurge

this is a really ugly and backwards function, but its presence will
prevent lots of broken gnulib software from trying to define its own
version of fpurge and thereby failing to build or worse.

include/stdio.h

@@ -158,6 +158,7 @@ int asprintf(char **, const char *, ...);
 int vasprintf(char **, const char *, va_list);
 void setlinebuf(FILE *);
 void setbuffer(FILE *, char *, size_t);
+int fpurge(FILE *);
 #endif
 
 #ifdef __cplusplus