There should be a +. The reason being that any system global CFLAGS vars will be added.
Case in point, RH loves to build in 386 compatibility mode. I haven't used a 386 in 5 years or more. I change my .rpmlib and also add a CFLAGS line in /etc/profile to make cpu=i686 and etc.
Without the += you will not pick up the environments CFLAGS and therefor get a build that is probably not optimized.
Regarding your problem, this shouldn't make any difference unless the software depends on some screwy compiler setting.
See http://jongriffin.com/static/linuxnotes for some settings you might want to try.
