Fixed prototype/definition conflicts with "traditional" C in skeleton at request...

Fixed prototype/definition conflicts with "traditional" C in skeleton at request of gcc developer.
Removed duplicate prototypes in gen.c, sym.c, main.c.
Added missing prototypes where needed.
All functions in skeleton follow ISO C style protos and defs, instead of BOTH ISO and new-style.
Skeleton now compiles cleanly under super-strict gcc flags.
Flex itself almost compiles cleanly under strict flags.