CMake Pthreads Fix

I updated both the main CMakeLists.txt and QGearsMain/CMakeLists.txt to
search for the pthreads library. I have Ogre built with threads enabled
but the linker was throwing an error when trying to link the executable.
It couldn't find pthread even though libprthread was installed.
Merge branch 'Draygera-master'
added config option for threading