[FIX] Using CMAKE_FIND_PREFIX_PATH in CMakeLists.txt.

It turns out that one should use CMAKE_FIND_PREFIX_PATH instead of CMAKE_FIND_ROOT_PATH for find_package() to work.
Merge pull request #102 from holtgrewe/hotfix/cmake_find_root_path

[FIX] Using CMAKE_FIND_PREFIX_PATH in CMakeLists.txt.