handle relative paths to parent directories (../foo/bar.tex). This
can be handled as was done in
https://svn.cs.uu.nl:12443/repos/trace/nix/trunk/make/lib/compile-c.sh.
The `copy-includes.pl' script is generic (not LaTeX specific), so it
should eventually replace most of compile-c.sh and any future
builders that have to set up a tree of symlinks to simulate the
original directory hierarchy of the sources.
svn path=/nixpkgs/trunk/; revision=3220
