Replace two deprecated parser generator directives

Replace two deprecated parser generator directives

There is a conflict between Flex's bison-bridge and Bison's api.prefix
options.  Apparently, the former was added without consulting the Bison
devs and requires YYSTYPE, which is not added to the header anymore by
the latter.  Instead, we just provide the proper definition of yyflex()
manually (as recommended by the Bison docs), so the option is not
required anymore.