Zope2: try to override only if chameleon is installed.

product/ERP5Type/configure.zcml

-<configure xmlns="http://namespaces.zope.org/zope">
+<configure xmlns="http://namespaces.zope.org/zope" xmlns:zcml="http://namespaces.zope.org/zcml" zcml:condition="installed chameleon">
   <include package="Products.PageTemplates" />
   <includeOverrides file="overrides.zcml" />
 </configure>

product/ERP5Type/overrides.zcml

-<configure xmlns="http://namespaces.zope.org/zope">
+<configure xmlns="http://namespaces.zope.org/zope" xmlns:zcml="http://namespaces.zope.org/zcml" zcml:condition="installed chameleon">
   <utility
     provides="zope.pagetemplate.interfaces.IPageTemplateEngine"
     component="zope.pagetemplate.pagetemplate.PageTemplateEngine"/>