System files and "make distcheck"

[cwr]

I'm building a package which on installation writes to a file used
to configure the GUI - this file is always in /etc, no matter where
the package is installed.  When installing, that's not a problem,
since installation is done as root, however I haven't found a way
of letting me run "make distcheck" without error, since that is done
as an ordinary user.

The entry in the main Makefile.am is approxmately:
install-data-local:
        grep xxx /etc/yyy || sed zzz /etc/yyy

The problem is to avoid this set of commands being run when
distcheck is the target.  I've tried using DISTCHECK_CONFIGURE_FLAGS,
but the main Makefile is written before this variable is valid.  This
problem must have ocurred before - what's the usual solution?

Thanks - Will