Note: In order to understand which versions of Apache are supported on which brands/versions of Unix (Linux, Solaris, HP-UX, etc...) you should access the System Requirements area of the Esri website.
Instructions for integrating other versions of ServletExec and ArcIMS can be found here.
The instructions at this website are provided by New Atlanta Communications.
At the time of this writing, Esri does not provide its own steps for installation and configuration of ServletExec and ArcIMS on a Unix platform.
However, several Unix platforms are supported... so in the event that they add such steps later,
a listing of all such Esri knowledge base articles can be found here.