Further improvements and fixes to FindParticleUniverse.cmake

This adds missing HINTS that were supposed to be part of my latest PR (they enable Linux users to locate the headers and libs without pkg-config, which is particularly useful since we don't provide a pkg-config file for now).

Debug build is now possible on Linux, provided the debug library is put in $libdir/OGRE (where the release library would be located on a system-wide installation). Most Linux distros do not provide debug libraries in their packages, though.