Revert "Eliminate unused #define."

This reverts commit 2fdba82e4f07f5708bd1c7390ab66fcb9d54ee3a.

Fixes MLton/mlton#40 (MinGW build broken).
Update links to mingw binaries hosted on mlton.org.

The conversion and migration of mlton.org content purged the
Experimental attachments.  The mingw binaries that had been downloaded
from the Experimental page have been moved to
www.mlton.org/misc/mingw.
Merge pull request #51 from MatthewFluet/mingw

Updates for MinGW port