Assume that <poll.h> is available

* configure.ac (AC_CHECK_HEADERS): Remove poll.h and sys/poll.h.
* pathtrace.c: Include <poll.h> unconditionally.
* stream.c: Likewise.
[HAVE_SYS_POLL_H]: Compile unconditionally.
[!HAVE_SYS_POLL_H]: Remove.
diff --git a/pathtrace.c b/pathtrace.c
index 3946782..f6f0dc1 100644
--- a/pathtrace.c
+++ b/pathtrace.c
@@ -28,11 +28,7 @@
 
 #include "defs.h"
 #include <sys/param.h>
-#if defined HAVE_POLL_H
-# include <poll.h>
-#elif defined HAVE_SYS_POLL_H
-# include <sys/poll.h>
-#endif
+#include <poll.h>
 
 #include "syscall.h"