undid last change (my bad). Put necessary fix in libxslt/libexslt instead.

* include/libxml/xpathInternals.h: undid last change (my
  bad).  Put necessary fix in libxslt/libexslt instead.
* include/libxml/DOCBparser.h: put test for __GCC__ on
  warning directive (Bug 129105)
diff --git a/include/libxml/xpathInternals.h b/include/libxml/xpathInternals.h
index 37c46a5..75e5d74 100644
--- a/include/libxml/xpathInternals.h
+++ b/include/libxml/xpathInternals.h
@@ -153,8 +153,7 @@
  * Pushes the string @str on the context stack.
  */
 #define xmlXPathReturnString(ctxt, str)					\
-    (str) == NULL ? valuePush((ctxt), xmlXPathNewCString("")) :		\
-    		    valuePush((ctxt), xmlXPathWrapString(str))
+    valuePush((ctxt), xmlXPathWrapString(str))
 
 /**
  * xmlXPathReturnEmptyString: