How to set ONLY_FOR_ARCH for binary ports?

[Kövesdán Gábor]

Hello,

I haven't found this in Porter's Handbook. How should I set 
ONLY_FOR_ARCHS in a port that installs i386 binaries? The amd64 
architecture, and IIRC ia64 too, have compatibility layers for i386 
architecture, but it isn't necessarily installed. Can I set 
ONLY_FOR_ARCH to "i386 amd64 ia64"? Or is there any way to check whether 
the compatibility layer is available?

Thanks,

Gabor Kovesdan