Recreate hardlinks instead of duplicating files

[embray]

As noted at https://trac.sagemath.org/ticket/28695#comment:11 a Cygwin installation, including many of the packages installed with Sage (particularly executables included with GCC), install hardlinks for several files. It's hard to get an exact number, but it seems there are about 40MB worth of files that have at least one hardlink.

When bundled into the installer, and then unpacked, these just become copies instead, thus taking up extra space.

It should be possible to figure out which files have hardlinks and to install only one copy of each of them, then recreate the hardlinks at install time (similarly to how symlinks are handled by the installer).

It's not a huge amount of space being wasted so this is not a high priority, but it is one possible avenue for a bit of space-saving if needed.