fixed problem in xmlTextWriterVSprintf caused by misuse of vsnprintf added

* xmlwriter.c: fixed problem in xmlTextWriterVSprintf caused by
  misuse of vsnprintf
* configure.in, config.h.in: added a configuration check for
  va_copy and added a define for VA_COPY for xmlwriter.c fix
* parser.c: fixed problem with CRLF split between chunks (bug
  #319279) (fix provided by Brion Vibber)
diff --git a/config.h.in b/config.h.in
index 8484193..9b03c22 100644
--- a/config.h.in
+++ b/config.h.in
@@ -237,6 +237,9 @@
 /* Define to 1 if you have the <unistd.h> header file. */
 #undef HAVE_UNISTD_H
 
+/* Whether va_copy() is available */
+#undef HAVE_VA_COPY
+
 /* Define to 1 if you have the `vfprintf' function. */
 #undef HAVE_VFPRINTF
 
@@ -252,6 +255,9 @@
 /* Define to 1 if you have the `_stat' function. */
 #undef HAVE__STAT
 
+/* Whether __va_copy() is available */
+#undef HAVE___VA_COPY
+
 /* Name of package */
 #undef PACKAGE