HAVE_SYS_POLL_H: Define on MacOS, FreeBSD, NetBSD & OpenBSD

[helmesjo]

49

I verified this on MacOS (was required to build thrift package). Haven't verified any other, though in my builds FreeBSD fails with the exact same error as MacOS, and some quick googling suggests the sys/poll.h header exists on the other BSD derivatives as well.

• https://github.com/lattera/openbsd/blob/master/sys/sys/poll.h
• https://ftp.netbsd.org/pub/NetBSD/NetBSD-current/src/sys/sys/poll.h