fix for building with gcc-2.3.3

[Zack Weinberg]

Jeffrey A Law wrote:
> 
> Someone tried to use gcc-2.3.3 to bootstrap gcc-2.95.  It didn't
> work because the initializer syntax for is_idchar isn't groked by
> gcc-2.3.3.
> 
> I have *serious* reservations about the next for this code to be gcc
> version dependent.  I don't see that the benefit of static
> initialization in this case is worth the headaches it involves.
> But for now I'm not going to zap the gcc special initialization.

It's done that way because the code in a couple of critical paths
(read_and_prescan, cpp_get_token) improves significantly if those
tables are constant.

zw