Commit 6a61a8a5 (Honor module .def files with MinGW tools, 2011-02-21)
set CMAKE_LINK_DEF_FILE_FLAG to "-Wl," to enable passing .def files to
the linker on MinGW.  However, older GNU tools in the MSYS shell do not
know how to translate "-Wl,/c/..." to "c:/..." and complain that the
file does not exist.  Instead set the flag to just "" which tells CMake
it can pass the file through the front-end with no special flag.