In 94cee4a4 I overlooked that config.h
unconditionally sets _FILE_OFFSET_BITS, so it cannot be used to detect
MinGW W64 environment. It looks like Emacs folks already found
solution; instead of _FILE_OFFSET_BITS use _W64 as suggested in
http://lists.gnu.org/archive/html/emacs-devel/2013-03/msg00723.html