default_variants per platform

[Anders F Björklund]

Is it possible to set "default_variants" per platform ?

Basically, I want to set +aqua for platform macosx,
and +x11 for platform puredarwin or platform freebsd.
I tried the most straight-forward approach, wrapping
default_variants in platform, but it didn't work...

For now I made due with "variant aqua requires macosx".

I also seemed to have the misguided notion that the
default variants would somehow go away when actually
providing some variants ? Not so, I had to give an
explicit "+x11 -aqua" to get rid of the defaults...

--anders