make get_build generic and safety

Created by: ghost

get_build was creating new Build instance with path.empty() = true by using its default constructor . if we use path.empty() for new CMakeBuild instance we will get the same result and don't need to mess with type hierarchy(lCMake and Project::CMake was different classes., Project::Build and Project::CMake castings are dangerous too). It will be great to provide the same functionality for MakeBuild, SconsBuild and so on in the future. And all internals in make.h, scons.h the same way as cmake.h.