added explanations on how to build on Windows 64-bit systems.
added required changes to _mingw.h to do so
added libcrtdll.def file in order to create a libcrtdll.a library in order to be able to link.
added more details and support for building on Windows based on a new fresh install on another Windows machine.
Merge pull request #5 from philippeback/master

added explanations on how to build on Windows 64-bit systems.