Emulate poll() by means of select() for 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(). To use it, set _WIN32_WINNT accordingly (i.e., < 0x600, which would be vista).