Build doc incorrect

http://doc.qt.nokia.com/qt3d-snapshot/qt3d-building.html#building-qt-3d

The make install step for the build Qt from source causes it to be placed into /usr/local/Trolltech/version/ and then the private headers are not available in that tree, so the remaining build instructions fail.

Note that this does not happen if the configure line is done with the -nokia-developer option, however the instructions just show building with ./configure -opengl, and for general use the -nokia-developer option seems wrong.

(Reported by Adam Weinrich)