It would be nice to have a ./configure option for xdm to specify a different path for xrdb, in case xrdb is installed in a different prefix.
Fixed in git master:
Author: Alan Coopersmith <email@example.com>
Date: Wed Jul 25 17:00:39 2007 -0700
Bug #10530: xdm's path to xrdb could be configurable at compile time
Add --with-xrdb=path & --with-default-session=command configure options
X.Org Bug #10530: <https://bugs.freedesktop.org/show_bug.cgi?id=10530>