Some of the cheaper or newer SSL suppliers will require this to work properly (otherwise you may be prompted that the cert is invalid when it's not the case but it will certainly scare off your users!).
In the Apache vhost conf for the domain here is what you add:
SSLCACertificateFile /path/to/your/cafile.pem
Here is a full example of an SSL Vhost config in Apache using a CA Certificate file
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