This variable identifies the routine to be used as the "sub" interface routine
within a DLL. It applies only to Windows systems. Set DLL_SUB_INTERFACE to the name of the routine you want to use. This name may be "sub"
or any name you choose. The runtime checks DLL_SUB_INTERFACE when a DLL is loaded. You may change its value afterwards without
any effect on DLLs that have already been loaded.
If DLL_SUB_INTERFACE is blank (default), then the runtime does not look for a "sub"
interface routine in a called DLL.