Option to create a single CMakeLists.txt

[sergio-eld]

Given a .sln file and a legacy project, a repo maintainer would not want to accept any commits with generated CMakeLists.txt files in each subfolder.
A non-invasive and potentially less time consuming would be to add an option to create a single (though large) cmake file with a possibility to override path to the source root dir. This way one could use cmake to build and customize visual-studio-based legacy project without modifying the source tree/repo/gitignore.