The ?= assignment of CFLAGS (not present in earlier lua5x patches) results in MYCFLAGS, passed from the port makefile, never getting assigned to CFLAGS during the lua build. The result is a copy of lua that has none of the posix support functions in it.
CFLAGS set in the outer environment by the user or make.conf get passed in via the MYCFLAGS mechanism, so remove the -O2 and let the outer environment control optimization level.