Add Darwin/Mac OS X support to the build system.

diff --git a/include/config.h.in b/include/config.h.in
index feaf9e1..3db9348 100644
--- a/include/config.h.in
+++ b/include/config.h.in
@@ -248,6 +248,9 @@
 /* Define to 1 if you have the `ossaudio' library (-lossaudio). */
 #undef HAVE_LIBOSSAUDIO
 
+/* Define to 1 if you have the `poll' library (-lpoll). */
+#undef HAVE_LIBPOLL
+
 /* Define to 1 if you have the `resolv' library (-lresolv). */
 #undef HAVE_LIBRESOLV