Haskell backend: happy does not accept empty token list, so bnfc should check for it

[andreasabel]

This (evil) grammar fails with a parse error in the Par.y file raised by happy:

A. S := T;
B. T := S;

happy does not like empty %token list.

[bafain]

Although the user guide says that the %token directive is mandatory, happy 1.19.9 does not complain if we omit it.

[andreasabel]

In case of no tokens, happy now does not output a %token declaration: f42310d.
Works for Haskell and Java backends now.

C/CPP: bison complains:

bison -t -pIssue213 Issue213.y -o Parser.c
Issue213.y: warning: 3 nonterminals useless in grammar [-Wother]
Issue213.y: warning: 3 rules useless in grammar [-Wother]
Issue213.y:68.8: fatal error: start symbol S does not derive any sentence
 %start S
        ^

I think this issue is not worth pursuing further.

[andreasabel]

It is easy to insert a check whether any terminals appear in the grammar.