gst/parse: define pure-parser depending on bison version

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.