Logged In: YES 
user_id=21627

Yes, setting CFLAGS appears to be the only way to achieve that.

There is also the issue of the user setting CFLAGS before
configure starts; traditionally, these should get
incorporated into both configure tests, and the final build
process. So something like

# early
BASECFLAGS=$CFLAGS

# after each block changing either BASECFLAGS or OPT
CFLAGS=$BASECFLAGS $OPT

might work (unless the user selects a flag that contradicts
with one set in BASECFLAGS)