Revert "Don't create recursive symlink, but reproduce glib header structure"

This somehow caused Xcode to always build almost the entire project
from scratch, as if it had been cleaned before. I'll have to investigate
this further, but it is not currently a priority.

Ref: MacIrssi/MacIrssi#61

This reverts commit d51cb5e5a78c1d1f156aa6edc7a377c71c42bef4.