CMake: Configure our .y files for different Bison/YACC flavors.
Bison 3.4 and later generate deprecation warnings for the "%pure-parser" directive. As https://git.savannah.gnu.org/cgit/bison.git/tree/NEWS says,
** Deprecated features
The %pure-parser directive is deprecated in favor of '%define api.pure' since Bison 2.3b (2008-05-27), but no warning was issued; there is one now. Note that since Bison 2.7 you are strongly encouraged to use '%define api.pure full' instead of '%define api.pure'.
Rename our .y files to .y.in, and modify FindYACC.cmake to detect newer versions of Bison and configure our .y files with "%pure-parser" or "%define api.pure" as needed.