build env vars

Hi all,

I first want to make it clear that I am not asking for "support" but
just asking if someone can point me in the right direction.¨

I've been playing with compiling the system with CPU specific flags for
example:
COPTS += -march=znver1 -mtune=znver1
CXXOPTS += -march=znver1 -mtune=znver1

Which for the base system seems to be fine so far, but for some ports
such as
devel/gcc/*
math/cblas

it seems that the compiler/linker (or whatever beyond my knowledge)
fails with CPU instructions that don't exist. Such as this:

Error: no such instruction: `vmovsd %xmm0,8(%rsp)'

This is fine, I am just trying to learn, but what I would like is if
someone can point me in the right direction to specify global
COPTS/CXXOPTS/CFLAGS/CXXFLAGS/etc but unset them for specific ports.

Or.. How to let these builds know that these instructions do exist
would be even more nice.. :-)

Any insight?

Thanks,
Justin