Added support of __thr_errno for reentrance on UnixWare.

diff --git a/include/acconfig.h b/include/acconfig.h
index 716085f..338f34a 100644
--- a/include/acconfig.h
+++ b/include/acconfig.h
@@ -45,6 +45,9 @@
 /* Define if libc uses ___errno for reentrant errno */
 #undef HAVE___ERRNO
 
+/* Define if libc uses __thr_errno for reentrant errno */
+#undef HAVE__THR_ERRNO
+
 /* Define if all debug messages are to be compiled out */
 #undef NO_DEBUG_MSGS