Emulate poll() by means of select() for to support Windows XP

WSAPoll() as an equivalent to POSIX poll() was only introduced with Windows Vista, WinSock from previous Windows versions doesn't have it. Even though you can compile and link on Windows XP, you can't run LCM if it uses WSAPoll. The patch introduces a poll() that implements this functionality by means of select().

The alternate poll() implementation is used when _WIN32_WINNT < 0x600, one could think about changing the Visual Studio project files to default to that so that resulting Windows binaries are always compatible with Windows XP and newer.