xa65 CMake Integration + include guard for "unistd.h" on non-nix-systems (reloc65.c)

[St0fF-NPL-ToM]

• added non-nix include guard (like found in f.e. file65.c) to reloc65.c
• created a CMake Integration for xa: CMakeLists and folder "cmake" -> make xa buidable using CMake -> allow to install locally after build -> create a CMake Package "XA65" for use by other CMake projects (create/manage XA65 targets) -> on Windows: packaging also works, a Windows installer can be produced easily for Deployment

Documentation is currently found inside all the source files. We'll have to discuss the form / destination of docs regarding above functionality.