[Libreoffice] Don't harcode theme list

[Sebastian Spaeth]

This patch to configure.in/set_soenv introduces the option
--enable-theme="theme1 theme2..." (with all available themes selected by
default).

set_soenv sets the environment variable ENABLE_THEME to the list of
themes. This part wrks (although configure could/should check against invalid
values being passed in).

THen packimages/makefile is modified in patch2, to use this passed in
list of themes to process rather than hardcoding a list of themes. This
should work but it untested so far.

The only piece missing is to get SCP2 to only install the selected
themes, I have not tackled this, but I might also not have time to do
so. So here you go for inclusion and further refinement.