windows: Include proper GUID defining header to fix build.

The initguid.h header must appear early or it does not have
an effect, at least on cross-build from Linux.
windows: Add -lsetupapi when building for WIN32.
windows: -no-undefined is required to make a DLL.
README: Mention that only MinGW-w64 works (but not MinGW).
README: Add URL to the libserialport website.
Add SP_MODE_READ_WRITE enum value.