No you can't. The reason is because the CFLAGS are up to the package maintainer, since there have been many bug reports in the past because people used way too aggressive CFLAGS causing the compiles to fail, or weird behavior in the programs.
if you want to mess with CFLAGS, you might want to take a look at Darwinports, which is based on the FreeBSD ports system, and is very similar to Gentoo's portage.