removed a couple of warning try to solve the linking problem on platforms

* catalog.c xmlcatalog.c: removed a couple of warning
* xpath.c: try to solve the linking problem on platforms
  needing trio to compile
Daniel
diff --git a/xmlcatalog.c b/xmlcatalog.c
index c2cd52f..b293ec3 100644
--- a/xmlcatalog.c
+++ b/xmlcatalog.c
@@ -12,6 +12,10 @@
 #include <stdio.h>
 #include <stdarg.h>
 
+#ifdef HAVE_STDLIB_H
+#include <stdlib.h>
+#endif
+
 #ifdef HAVE_LIBREADLINE
 #include <readline/readline.h>
 #ifdef HAVE_LIBHISTORY