(Because it could be bison -y and flex.)
The build uses yacc and lex so USE_GNU_TOOLS. (Note that
USE_GNU_TOOLS is broken, so now on NetBSD this will use bison -y instead
of native yacc.)
Also set YACC in MAKE_ENV.
This is partial fix for PR #pkg/29388. (The remaining problem
is that mk/tools.mk assumes that yacc is already installed
so YACC is never set to "bison -y" as needed.)