Re-examined the problems of configuring a "minimal" library.
Synchronized the header files with the library code in order
to assure that all the various conditionals (LIBXML_xxxx_ENABLED)
were the same in both.  Modified the API database content to more
accurately reflect the conditionals.  Enhanced the generation
of that database.  Although there was no substantial change to
any of the library code's logic, a large number of files were
modified to achieve the above, and the configuration script
was enhanced to do some automatic enabling of features (e.g.
--with-xinclude forces --with-xpath).  Additionally, all the format
errors discovered by apibuild.py were corrected.
* configure.in: enhanced cross-checking of options
* doc/apibuild.py, doc/elfgcchack.xsl, doc/libxml2-refs.xml,
  doc/libxml2-api.xml, gentest.py: changed the usage of the
  <cond> element in module descriptions
* elfgcchack.h, testapi.c: regenerated with proper conditionals
* HTMLparser.c, SAX.c, globals.c, tree.c, xmlschemas.c, xpath.c,
  testSAX.c: cleaned up conditionals
* include/libxml/[SAX.h, SAX2.h, debugXML.h, encoding.h, entities.h,
  hash.h, parser.h, parserInternals.h, schemasInternals.h, tree.h,
  valid.h, xlink.h, xmlIO.h, xmlautomata.h, xmlreader.h, xpath.h]:
  synchronized the conditionals with the corresponding module code
* doc/examples/tree2.c, doc/examples/xpath1.c, doc/examples/xpath2.c:
  added additional conditions required for compilation
* doc/*.html, doc/html/*.html: rebuilt the docs
diff --git a/tree.c b/tree.c
index 805db66..31511c5 100644
--- a/tree.c
+++ b/tree.c
@@ -1721,7 +1721,8 @@
 }
 #endif /* LIBXML_TREE_ENABLED */
 
-#if defined(LIBXML_TREE_ENABLED) || defined(LIBXML_HTML_ENABLED)
+#if defined(LIBXML_TREE_ENABLED) || defined(LIBXML_HTML_ENABLED) || \
+    defined(LIBXML_SCHEMAS_ENABLED)
 /**
  * xmlNewProp:
  * @node:  the holding node
@@ -2963,7 +2964,8 @@
     return(elem);
 }
 
-#if defined(LIBXML_TREE_ENABLED) || defined(LIBXML_HTML_ENABLED)
+#if defined(LIBXML_TREE_ENABLED) || defined(LIBXML_HTML_ENABLED) || \
+    defined(LIBXML_SCHEMAS_ENABLED)
 /**
  * xmlAddPrevSibling:
  * @cur:  the child node