Restricting Doxygen indexing to particular LibreOffice modules

I have committed a code change to the LibreOffice tree to allow developers to specify what modules they want Doxygen to index. I have done this because indexing every module is quite time consuming, and you may only want to review a subset of modules (in my case I am interested primarily in the vcl module).

To index specific modules only, you now export the $INPUT_PROJECTS environment variable and run make docs. For example, if you want to index only the vcl and svtools modules, you would do the following:

chris@libreoffice-ia64:~/repos/libreoffice$ export INPUT_PROJECTS="vcl svtools"
chris@libreoffice-ia64:~/repos/libreoffice$ make docs
chris@libreoffice-ia64:~/repos/libreoffice$ unset INPUT_PROJECTS