gnu: texlive-libkpathsea: Tweak "texmf.cnf" variables.

* gnu/packages/tex.scm (texlive-libkpathsea): Use XDG variables instead of
HOME for cache and user configuration.  Re-instate TEXMFLOCAL, which may be
necessary for external TeX installations.
(texlive-luatex): Re-instate TEXFLOCAL.
* gnu/packages/algebra.scm (gap):
* gnu/packages/assembly.scm (asl):
(yaehmop):
* gnu/packages/bioinformatics.scm (discrover):
* gnu/packages/chez.scm (chez-scheme-for-racket):
* gnu/packages/engineering.scm (fastcap):
* gnu/packages/fsf.scm (book-faif):
* gnu/packages/maths.scm (hypre):
(stex-boostrap):
(suitesparse-amd):
(suitesparse-camd):
(suitesparse-cholmod):
(suitesparse-klu):
(suitesparse-ldl):
(suitesparse-mongoose):
(suitesparse-spex):
(suitesparse-spqr):
(suitesparse-umfpack):
* gnu/packages/ocaml.scm (ocaml-bibtex2html):
(unison):
* gnu/packages/python-xyz.scm (python-nbconvert):
* gnu/packages/statistics.scm (r-with-tests)[arguments]: Set TEXMFVAR to
a writable location.

Change-Id: I3d2d069cf41a92214d9d6fb7f4d54fc81fe6db7a

gnu/packages/chemistry.scm

@@ -886,6 +886,7 @@ emphasis on quality rather than speed.")
                   (("file=dihedral\\.eps")
                    "file=figs/dihedral.eps"))
                 (setenv "FORCE_SOURCE_DATE" "1")
+                (setenv "TEXMFVAR" "/tmp")
                 (invoke "latexmk" "-pdf" "bind_manual.tex"))))
           (add-after 'install 'install-eht-parms
             (lambda _