k-order DLL: simplify and better document correspondence between Dynare and Dynare++ variable orderings

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We now use a symbolic link farm to make source file accessible from build
directories.

Closes #466

It looks like in Emacs 26, the semantics of .dir-locals.el has changed:
overriding the default value of indent-tabs-mode in makefile-mode no longer
work. Therefore we no longer set a default value and rather set the variable on
a per-mode basis.

By the way, remove the setting for fill-column.

By the way, drop ax_boost_base.m4 which is no longer used.

By the way, enable the option in example1_korder.mod (it does not work, but no
harm since it’s not in the testsuite).

Thus we can remove unfolding code from k_order_pert.m.

We still need to properly define available memory.

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