[osg-submissions] API configurations in a separate Config include file

[Paul Martz]

I was surprised to see a change with such widespread ramifications discussed
only on osg-submissions, so I'm cross-posting.

I think I've now waded through enough of the posts in this thread to
understand why this was done, and how to cope with the fact that OSG header
files are now in the build directory.

Mathias, could you take the time to enter a FAQ on the wiki regarding this?
I foresee many posts when people upgrade to the latest OSG of the form: "why
can't my application find the Config header?". Users will need to know the
following:

 * OSG now generates headers in the build tree. If your app links with OSG
from a source/build tree, your app will now need to look for headers in
multiple locations.

 * You can get around this by installing OSG. You can customize the install
location with an OSG cmake variable.

 * Any other info you think is relevant.

A FAQ on the wiki would be better than having people search the
osg-submissions archives.

Thanks,
   -Paul