Atualize para o Apache 2.3.6 ou posterior.
Em apache.org :
IncludeOptional Directive
Description: Includes other configuration files from within the server configuration files
Syntax: IncludeOptional file-path|directory-path|wildcard
Context: server config, virtual host, directory
Status: Core
Module: core
Compatibility: Available in 2.3.6 and later