Only build 64-bit on Leopard when requested

Leopard 64-bit is still experimental and error-prone; users will need to
opt into this. It's too cruel to make the default right now.
ENV: enable ld64 on Leopard when available

Leopard's ld is very, very badly broken, causing it to fail to link
valid 64-bit builds. Our ld64 is newer and fixes this particular bug,
so it should get enabled in every build on Leopard-64 as long as it's
installed.