Did you spot the APPEND system option? And his compatriot INSERT? Both are extremely useful when you have a chain of configuration files. By default, the most recent specification of a system option (such as SASAUTOS) is the one that SAS will use. INSERT and APPEND allow you to add values to these options, either at the beginning (INSERT) or at the end (APPEND).
The full list of options to which INSERT and APPEND can be applied is: FMTSEARCH, HELPLOC, MAPS, MSG, SAMPLOC, SET, SASSCRIPT, SASAUTOS, or SASHELP.
So, for instance, if you have an installation-wide configuration file that specifies the standard SAS macro library concatenation, and you have an application-specific configuration file that needs to specify the application-specific macro library, you no longer need to repeat the standard SAS libraries in the app-specific config. Instead, you can just INSERT the app-specific library at the head of the concatenation and thereby de-couple your app-specific config file from the installation-wide config.
The SAS 9.2 Language Reference: Dictionary provides the low-down detail.