Use poll timeout workaround on macOS only

Other systems implement proper poll according
to POSIX and return when the pipe is closed.

libs/pbd/system_exec.cc

@@ -836,16 +836,27 @@ SystemExec::output_interposer ()
 	for (;fcntl (rfd, F_GETFL) != -1;) {
 		r = read (rfd, buf, BUFSIZ - 1);
 		if (r < 0 && (errno == EINTR || errno == EAGAIN)) {
+
+#ifdef __APPLE__
 again:
+#endif
 
 			/* wait till ready to read */
-
 			struct pollfd pfd;
 
 			pfd.fd = rfd;
 			pfd.events = POLLIN|POLLERR|POLLHUP|POLLNVAL;
 
+#ifdef __APPLE__
+			/* on macOS poll() will not return when the pipe
+			 * is closed in an EOF state.
+			 * Work around with a timeout and fail next time
+			 * when with POLLNVAL.
+			 */
 			int rv = poll (&pfd, 1, 1000);
+#else
+			int rv = poll (&pfd, 1, -1);
+#endif
 
 			if (rv == -1) {
 				break;
@@ -859,10 +870,12 @@ again:
 				/* back to read(2) call */
 				continue;
 			}
+#ifdef __APPLE__
 			if (rv == 0) {
 				/* Timeout, poll again */
 				goto again;
 			}
+#endif
 		}
 		if (r <= 0) {
 			break;