Silence Bison deprecation warnings

Bison >=3.0 issues warnings about

    %name-prefix="base_yy"

instead of the now preferred

    %name-prefix "base_yy"

but the latter doesn't work with Bison 2.3 or less.  So for now we
silence the deprecation warnings.

Back-patch to 9.2 and 9.3 -- the newer branches already have this fix.

Author: Peter Eisentraut
Discussion: https://postgr.es/m/677.1483384145%40sss.pgh.pa.us