Changes to make `clex' compile again.

The basic and portable solution that I know of is simply to put various
"extern" declations in the driver C code.  This is inelegant, but it is
simple and doesn't assume anything about whether or not you are using
flex.

Aside: The `ylwrap' tool from Automake 1.13.2 apparently doesn't know
anything special about the --header-file option to flex.  I was a bit
surprised by this, since it *does* have special knowledge for handling a
similar flag for bison (-d).  See the Automake manual for more info.
Googling about, I see that people have proposed patches to add support
for --header-file.  I don't know why those patches have not been
accepted.

If extern declarations become tedious, we can try doing something else,
at the possible cost of requiring flex (not lex).