determine correct way to specify positron/app/spidernode subconfigure dir

[mykmelez]

old-configure.in includes the comment:

# TODO: remove this (and uncomment the equivalent in positron/configure.in)
# once https://bugzilla.mozilla.org/show_bug.cgi?id=1292653 has been fixed.
AC_OUTPUT_SUBDIRS(positron/app/spidernode)

And positron/configure.in similarly says:

# TODO: uncomment this (and remove the equivalent in old-configure.in)
# once https://bugzilla.mozilla.org/show_bug.cgi?id=1292653 has been fixed.
# AC_OUTPUT_SUBDIRS(positron/app/spidernode)

But https://bugzilla.mozilla.org/show_bug.cgi?id=1292653 was resolved wontfix, because:

The "use stuff from $MOZ_BUILD_APP/configure.in" is legacy and kept for comm-central. It is not meant to be used by anything new. This is why there is a ".." hardcoded there to make this thing work for comm-central, and that is why it doesn't work for you because it's not meant to.

So we should determine the correct way to specify that positron/app/spidernode is a subconfigure directory.