Look for execinfo.h header

git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4057 d4fa192b-c00b-0410-8231-f00ffab90ce4

configure.ac

@@ -84,6 +84,7 @@ AC_CHECK_HEADERS([sys/socket.h])
 AC_CHECK_HEADERS([sys/statvfs.h])
 AC_CHECK_HEADERS([sys/vfs.h])
 AC_CHECK_HEADERS([endian.h])
+AC_CHECK_HEADERS([execinfo.h])
 AC_CHECK_HEADERS([netinet/in.h])
 AC_CHECK_HEADERS([pthread_np.h])
 AC_CHECK_HEADERS([stddef.h])