Add Bison-related configure tests.

Recent versions of Bison have made a bunch of incompatible behaviour
changes.  Implement configure tests to identify some issues, so that
build compatibility with multiple versions can be achieved.

As part of this, we introduce a basic macro for finding Bison in the
first place, which will verify support for "modern" features like %code
blocks which are needed by all my Bison-using projects.