%define token.prefix.

Provide a means to add a prefix to the name of the tokens as output in the
generated files.  Because of name clashes, it is good to have such a
prefix such as TOK_ that protects from names such as EOF, FILE etc.
But it clutters the grammar itself.

	* data/bison.m4 (token.prefix): Empty by default.
	* data/c.m4 (b4_token_enum, b4_token_define): Use it.
	* data/lalr1.cc (b4_symbol): Ditto.