Custom compiler/{C,CXX,F}FLAGS and /etc/make.conf - how to?

[Vladimir Kushnir]

Dear ALL,
The subject says it all. I'm trying to push out of my box every ounce of 
performance, perhaps even with (yet experimental) path64 compiler. So my 
question is as simple as that: what is the precise spell to put in 
make.comf to get (while not disrupting the ports infrastructure!) 
-march=amdfam10 if compiler is lang/gcc46 and -march=barcelona for path64 
(perhaps yet another flags as well if toolchain supports them)?

TIA,
Vladimir