Porting libxml2 on zOS encoding of code

First set of patches for zOS
- entities.c parser.c tree.c xmlschemas.c xmlschemastypes.c xpath.c xpointer.c:
  ask conversion of code to ISO Latin 1 to avoid having the compiler assume
  EBCDIC codepoint for characters.
- xmlmodule.c: make sure we have support for modules
- xmlIO.c: zOS path names are special avoid dsome of the expectstions from
  Unix/Windows

xmlmodule.c

@@ -8,6 +8,11 @@
  * http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html
  */
 
+/* In order RTLD_GLOBAL and RTLD_NOW to be defined on zOS */
+#if defined(__MVS__)
+#define _UNIX03_SOURCE
+#endif
+
 #define IN_LIBXML
 #include "libxml.h"