After release bison 2.5 the declaration %pure-parser was deprecated
in favor of %define api.pure

Nonetheless, until bison 3.4, the declaration was treated as backward
compatibility, but now bison shows a warning:

  warning: deprecated directive, use ‘%define api.pure’

The patch's approach is to handle both directives according with the
used bison's version, by string replacement at source configuration
stage.