push-pull syntax varies between Bison versions

[Rik]

2/28/13

John,

The new directive for a push-pull parser is currently

%define api.push-pull both

which is appropriate for bison versions >= 2.5.

The original syntax that they introduced in 2.4 was

%define api.push_pull "both"

which has underscores and quoting.  It looks like an autoconf test is in
order to verify which syntax the host is using.

--Rik