Documentation build target is not automatic

[Paul Martz]

Hi all -- Now that we have osgBullet's CMake pretty much working again, I
thought it was time to break it again. :-)

For both osgWorks and osgBullet on trunk, the "Documentation" build target is
now created automatically if Doxygen is found, and not created if Doxygen isn't
found. There is no longer an explicit "*_BUILD_DOCUMENTATION" CMake variable.
The benefit of this is that it's one less variable occupying space in the
cmake-gui. On the other hand, if anyone has problems with this change, please
let me know and I'll back it out.

Thanks,

--
-Paul Martz Skew Matrix Software
http://www.skew-matrix.com/