Fix cmake build instructions for missing package

* only run qt-specific commands if required qt package was found
* else QT4_WRAP_CPP and QT4_WRAP_UI will be skipped which does not
  matter since the target won't be included anyway (due to missing
  required package Qt/etc)
Fix voxdesk lighting

* glGetLightfv() returns position in eye coordinates, therefore
  transform to volume coordinates
* calculate N, L and H for blinnPhong shading correctly
* fix tex2vol lookup
rename misleading parameter

* gradient() takes texture coordinates, no volume position
Compile fix: missing include

* include <cstring> for memset()
Do not ignore attenuation settings for lighting

* settings do not depend from clipping objects