[O] Location of OpenDocument style files should be configurable

[Ulrich Mueller]

Hi,
In Gentoo we have an issue with the location of the OpenDocument style
files, reported to us in bug 396269 [1].

When org-mode is installed as a separate Gentoo package, its lisp
files are installed in /usr/share/emacs/site-lisp/org-mode/.
According to our policy, non-lisp files would go to
/usr/share/emacs/etc/org-mode/. We would pass this as the "etcdir"
parameter to "make install", so the odt style files would be installed
in stylesdir = /usr/share/emacs/etc/org-mode/styles/. 

Now org-odt.el currently defines org-odt-styles-dir in a defconst,
and it searches only in ../etc/styles/, ./etc/styles/, and ./etc/org/
relative to lispdir. Obviously it cannot find the files in the Gentoo
location.

Now my question, could the defconst be changed to a defvar, so that we
can specify the location in our site-start file? Or, preferably, could
the etcdir specified in "make" command be honoured in the lisp code?

Ulrich

[1] <https://bugs.gentoo.org/show_bug.cgi?id=396269>