recipes/auctex.rcp: Set correct dirs.

If `packagelispdir` and `packagedatadir` are not set then AUCTeX expects
tex-site.el to be in a directory above the actual auctex dir and sets
the variables `TeX-data-directory` and `TeX-lisp-directory` to incorrect
values.  This will prevent such things as correct loading of the styles/
directory.

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For darwin backlash (\`) is used.  I'm not sure if this still works properly (no osx/darwin to test it) because when I tried it for the `package*dir` parameters it seems the configure script got a literal \`pwd\` instead of the result of `pwd`. Maybe `(concat "--with-...=" default-directory)` should be used instead to make it independent of shell interpolation all together.