Fix CFLAGS handling in lua53 patches.

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.

Approved by: mat(mentor)
Differential Revision: https://reviews.freebsd.org/D5256