Fix xmlTextWriterWriteElement when a null content is given
Don't use xmlValidateName() when not available.

Fix compilation with minimum and debug.
Legacy needs xmlSAX2StartElement() and xmlSAX2EndElement().

Fix compilation with minimum and legacy.
Fix compilation with minimum and schematron.

Add a hard dependancy on tree.
Disable write and close callbacks when output is disabled.
Compile out use of xmlValidateNCName() when not available.

Fix compilation with minimum and valid.
Fix compilation with minimum and xinclude.

xinclude needs xmlAddNextSibling().
Compile out use of xmlLocationSetPtr when xptr is disabled.
Include xpath header.
Avoid a possible NULL pointer dereference

For https://bugzilla.gnome.org/show_bug.cgi?id=708355