Fix Windows build.
Fix Mac OS X build (untested).
linux_termios: fix return types to actually match returned values

This fixes the following warning :

linux_termios.c: In function 'get_termios_get_ioctl':
linux_termios.c:42:2: warning: overflow in implicit constant conversion [-Woverflow]
  return TCGETS2;
  ^

This fixes bug #396.
linux: remove readlinkat() dependency
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.