unset or change CPATH/LIBRARY_PATH for build in a portfile

[christos koninis]

Hello all,

Is there any way to unset or change the CPATH/LIBRARY_PATH environment
variables in a portfile during the build?
I have tried

build.env-delete        CPATH
build.env               CPATH=something

but they don't seem to be working.

Thanks,

Christos Koninis