To comply with the latest POSIX standard, in Yacc compatibility mode
(options `-y`/`--yacc`) Bison now generates prototypes for yyerror and
yylex. In some situations, this is breaking compatibility: if the user
has already declared these functions but with some differences (e.g., to
declare them as static, or to use specific attributes), the generated
parser will fail to compile. To disable these prototypes, #define yyerror
(to `yyerror`), and likewise for yylex.
refer: https://git.savannah.gnu.org/cgit/bison.git/tree/NEWS
GNU Bison 3.8
it turns out that we don't need to tell lex we don't plan to use unput;
we can just.... not use unput. however, if we're flex, use the command
line option if it's available, to avoid warnings
