Various fixes for cross-compilation

These commits improve the build system in various ways to improve the cross-compilation use-case
I am not sure to understand what you wanted to do here. The find_path should always succeed as this file is part of a submodule of haka. Also with LUAC=no, the compilation should not do anything so the file will just be install in the final package. That was meant to allow haka user to create tests if needed.

Maybe adding a build variable for that would be better or just to let it the way to was ?