Improve PU detection with cmake on Linux

This was tested on Mageia Cauldron. Pkg-config can be used to find the headers and libraries, but there is currently no pkg-config file provided with ParticleUniverse. The Find module using pkg-config was tested locally with a home made OGRE-ParticleUniverse.pc file:

```
prefix=/usr
exec_prefix=/usr
libdir=/usr/lib64
includedir=/usr/include

Name: OGRE-ParticleUniverse
Description: Particle Universe plugin for Ogre3D with community fixes
Version: 1.9.0

Requires: OGRE = 1.9.0
Libs: -L${libdir}/OGRE -lPlugin_ParticleUniverse
Cflags: -I${includedir}/OGRE/Plugins/ParticleUniverse
```

(Note that the paths have been created at packaging time and would be relative to the arch and version of the PU package).