Change prefix path

- FindHDF5.cmake: honors the ENV variable HDF5_ROOT -> removed from Prefix
- 64bit Ubuntu: if one uses self-compiled compilers and someone forgot to set
                the multiarch lib dir /usr/lib/x86_64-linux-gnu/ in the
                default compiler path we have to manually add this...
Merge pull request #19 from ax3l/fix/CmakePaths

Change prefix path
Add x86_64-linux-gnux as cmake prefix path
Merge branch 'master' of github.com:ComputationalRadiationPhysics/libSplash

Conflicts:
	CMakeLists.txt