https://sourceforge.net/tracker/?func=detail&aid=2178663&group_id=97492&atid=618177

diff -Naurp flex-2.5.35-orig/skel.c flex-2.5.35/skel.c
--- flex-2.5.35-orig/skel.c	2009-03-25 22:21:16.000000000 -0600
+++ flex-2.5.35/skel.c	2009-03-25 22:22:22.000000000 -0600
@@ -286,6 +286,7 @@ const char *skel[] = {
   "#include <errno.h>",
   "#include <cstdlib>",
   "#include <cstring>",
+  "#include <cstdio>",
   "/* end standard C++ headers. */",
   "%endif",
   "",