[XeTeX] Where is the configuration file for xdvipdfmx ?

[Ron Aaron]

I tried to find a file where I could tell 'xdvipdfmx' to always set
"-V6", but so far I cannot find it.

The other 'dvi...' files seem to have config information in /etc/texmf
(but modifying them doesn't affect xdvipdfmx).  Also, the docs for it
are pretty sparse.

Anybody know how to permanently affect the config so I don't have to use
'-output-driver=...' on xetex command line?

Thank you
Ron