Use the "-o" flag, rather than using the "-t" flag and redirecting the

Use the "-o" flag, rather than using the "-t" flag and redirecting the
standard output, in the rules to get Flex to produce scanner code; that
way, if Flex fails to run for some reason, we don't leave around a
zero-length or otherwise incorrect "XXX-scanner.c" file that might
keep a subsequent make from thinking it has to generate that file.

git-svn-id: http://anonsvn.wireshark.org/wireshark/trunk@1808 f5534014-38df-0310-8fa8-9805f1628bb7