IF(X64 AND NOT APPLE) does also apply for a 64bit linux system, so "-win64" is not appropriate here. If there is no real difference in the files, lets just use "3rdparty/assimp-3.0" for all platforms. Also, please include a readme-file in "3rdparty/assimp-3.0" describing how to download, build and install the library (e.g., copy files into ./libs directory).