configure.ac: Teach it about bison >= 3

When encountering bison >= 3, treat it as ok, always. This addresses
part of #49.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
lib/cfg-grammar.y: bison3 compatibility

In bison3, YYID() is gone. To make things easier, this patch adds a
compatibility macro in this case.

With the previous patches, this fixes #49.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>