... by implementing a fake ln in qmake.

symlinks are supported only since vista (we officially still support
xp), and even there are permission-restricted (MS being (rightfully)
afraid of symlink attacks). so we fake the links by copying the files
instead.

the previous hack was a bit naive, simply using cp/copy instead of ln.
this didn't work with relative paths, as real symlinks are resolved
against their parent directory, not the working directory of the "ln"
command. the new fake does this correctly.

Change-Id: Ia2f5d68a39d6ffcc8a4383f9d0fc63a9da0a05c3
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>