libre2c: added prefix 'TOKEN_' to all parser tokens to avoid name collisions.

libre2c: added prefix 'TOKEN_' to all parser tokens to avoid name collisions.

Apparently some versions of bison generate token definitions as macros, not
as enum members. If a bison-generated header is included into another file
that contains identifiers identical to the names of parser tokens, all such
identifiers are macro-substituted to a numeric constant. Adding a prefix to
token names reduces the chances of name collision.

This fixes bug #261: "lib/parse.h:49:15: 错误：expected unqualified-id before
numeric constant" reported by qidizi.