cmake: Check for unistd.h.

src/poll.c

@@ -115,10 +115,13 @@ static poll_fn ssh_poll_emu;
 #else /* _WIN32 */
 #include <sys/select.h>
 #include <sys/socket.h>
-#include <unistd.h>
 #include <sys/time.h>
 #endif /* _WIN32 */
 
+#ifdef HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+
 
 /*
  * This is a poll(2)-emulation using select for systems not providing a native