[CMake] why are includes not used from installed prefix?

[lux-integ]

I am a novice with cmake dabbling with KDE4  and a little with blender) (on 
X86_64(only) Linux. Some of our applications are installed in prefix /opt    
For KDE4 compilation for example  I noticed that libraries  are detected  ( 
reported correctly by cmake )  but  often 'make'  fails as  the includes 
seems to  'required'  in /usr.  

are these   errors  OR  methods?