revamped the elfgcchack.h format to cope with gcc4 change of aliasing

* doc/apibuild.py doc/elfgcchack.xsl: revamped the elfgcchack.h
  format to cope with gcc4 change of aliasing allowed scopes, had
  to add extra informations to doc/libxml2-api.xml to separate
  the header from the c module source.
* *.c: updated all c library files to add a #define bottom_xxx
  and reimport elfgcchack.h thereafter, and a bit of cleanups.
* doc//* testapi.c: regenerated when rebuilding the API
Daniel
diff --git a/tree.c b/tree.c
index 42795b5..ff47a51 100644
--- a/tree.c
+++ b/tree.c
@@ -7119,3 +7119,5 @@
     else xmlCompressMode = mode;
 }
 
+#define bottom_tree
+#include "elfgcchack.h"