Port small part of master's 8901f32e so future ports are easier.

Port small part of master's 8901f32e so future ports are easier.

* data/lalr1.cc (yy::parser::yysyntax_error_): Always add second
argument, but name it in the function definition only when
verbose error messages are enabled and it'll thus be used.
(yy::parser::parse): Update use of yysyntax_error_.