lily/parser.yy: Use %define api.pure instead of %pure_parser

lily/parser.yy: Use %define api.pure instead of %pure_parser

%pure-parser would apparently still be supported by Bison 3.0, but it
has been deprecated in Bison 2.6.2 already.  It is not quite clear
when the original %pure_parser (with underline instead of dash) has
become deprecated: Bison 2 supports it still, Bison 3 doesn't.


However, we should probably make sure that GUB is fine with this: I
don't know when the api.pure define was established and what Bison
version we have in GUB.

[dak, 2013-08-06 13:08:23]

Use %pure-parser in order not to be stalled on GUB updates