These were workarounds for a long-gone flex bug; all supported versions of flex emit an extern declaration as expected.
9a8e233 Remove a couple other vestigial yylex() declarations.
src/backend/utils/misc/guc-file.l | 3 ---
src/bin/psql/psqlscan.l | 2 --
2 files changed, 5 deletions(-)
Upstream: git.postgresql.org