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>
Makefile.am: bison >= 3 improvements

We add -Wno-yacc to AM_YFLAGS, to simplify the Makefile. This works for
all autoconf versions, with all bison versions, and we require bison
anyway. We also add -Wno-other there, to skip the bison warnings we
previously grepped out in Mk/lex-rules.am.

Grepping does not work anymore, as bison 3.0 uses multi-line messages,
so we get half of them anyway.

This addresses another 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>
Mk/lex-rules.am: Silence mkinstalldirs in silent mode

When in silent mode, make the mkinstalldirs calls silent too.

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