Add CMAKE_GNUtoMS option to convert GNU .dll.a to MS .lib

Teach the Windows-GNU.cmake platform file to look for Visual Studio
tools matching the target ABI.  Add an extra step to the link command
for shared libraries and executables that export symbols and on which a
new GNUtoMS property is set (initialized by the CMAKE_GNUtoMS option).
Tell the GNU linker to output a module definition (.def) file listing
exported symbols in addition to the GNU-format import library (.dll.a).
Pass the .def file to the MS "lib" tool to construct a MS-format DLL
import library (.lib).

Teach the install(TARGETS) command to install the MS import library next
to the GNU one.  Teach the install(EXPORT) and export() command to set
the IMPORTED_IMPLIB property pointing at the import library to use the
import library matching the tools in the importing project.