The option was added in bug 467583 so that both top-level and js/src can use the same $DIST directory. I actually removed the --with-dist-dir option given to js/src's subconfigure in bug 969164 because $DIST was deduced some other way in that case. Bug 1227385 exposes new problems with $DIST not having the right value when executing js/src/configure after bug 1224490, but I don't want to re-use --with-dist-dir to fix them. I'd rather remove the option altogether, it was never meant for broad consumption anyways, doesn't appear to be used by anything else. And there is no such flag in the top-level build system.
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