Windows: Enable MinGW and MSVC DLL interchangeability

Windows: Enable MinGW and MSVC DLL interchangeability

* Because we use the WINAPI calling convention, the def file MUST have the @n
  aliases. There is no way around this as MinGW's .o use decoration always
  for __stdcall, which can't be turned off.
* dlltool must therefore be invoked to create a proper import lib from the .def,
  using the --kill-at option.
* To do that, a CREATE_IMPORT_LIB autotools variable is introduced.
* Note: the .def file is currently maintained manually.