Use the GetShortPathNameW return value to allocate a buffer of the
required size.  The previous assumption that the "short" path will
always require less space than the original path is incorrect.

CMake-Issue: 15892
Inspired-by: Jan van Dorsten <dorsten@boehme-weihs.de>
Change-Id: I5845752710560b3e71a2f5eaf334dd676f5b3a9d