#296 Allow DOSBox to run without CD-ROM support

[Cameron Cawley]

On platforms like RISC OS where the SDL CD-ROM APIs are unsupported, this patch allows DOSBox to be run without it.

This serves as a more generic version of this patch: http://www.riscos.info/websvn/filedetails.php?repname=gccsdk&path=%2Ftrunk%2Fautobuilder%2Femulation%2Fdosbox%2Fsdlmain.p