hack based on Arjan van de Ven suggestion to reduce ELF footprint and

* elfgcchack.h doc/elfgcchack.xsl libxml.h: hack based on Arjan van de
  Ven suggestion to reduce ELF footprint and generated code. Based on
  aliasing of libraries function to generate direct call instead of
  indirect ones
* doc/libxml2-api.xml doc/Makefile.am doc/apibuild.py: added automatic
  generation of elfgcchack.h based on the API description, extended
  the API description to show the conditionals configuration flags
  required for symbols.
* nanohttp.c parser.c xmlsave.c include/libxml/*.h: lot of cleanup
* doc/*: regenerated the docs.
Daniel

libxml.h

@@ -49,4 +49,12 @@ extern int __xmlRegisterCallbacks;
  */
 void __xmlIOErr(int domain, int code, const char *extra);
 void __xmlLoaderErr(void *ctx, const char *msg, const char *filename);
+
+#ifdef IN_LIBXML
+#ifdef __GNUC__
+#ifdef linux
+#include "elfgcchack.h"
+#endif
+#endif
+#endif
 #endif /* ! __XML_LIBXML_H__ */