Since commit 024d05ad (Fix use of module .def files for MS tools,
2009-09-29) module .def files work for any platform that sets
CMAKE_LINK_DEF_FILE_FLAG correctly.  Set it in the Windows-GNU platform
information file to enable support with MinGW tools.  Also enable the
test added by commit 0db2c850 (Test use of module .def files for MS
tools, 2009-09-29) for MinGW and MSYS generators.