Add :ld64 dependency

This allows formulae which won't build with Tiger's ld to conditionally
request a dependency on the ld64 formula. This modifies the build
environment appropriately, and will only be active on Tiger.
cctools-headers 836
libunwind-headers 35.1
dyld-headers 210.2.3
cctools-headers: Update comment
ld64 97.17
ld64: fails_with :gcc_4_0

Fixes #20.
ld64: style nits
ld64: force 32-bit on Leopard
ld64 builds 64-bit after all

This reverts commit 0c6712af6cab89076bce113bbb563688ab9a418f.