Two branches in that repository are crucial: One for the stable versions (master branch) and an other one for current developments (developer branch).

To build alien you need Microsoft Visual Studio 2019. You find the solution file in msvc/alien/alien.sln.
The following third-party libaries are necessary and should be installed:

Provided that an appropriate build system is set up, it is in principle possible to compile the sources on other platforms (e.g. using gcc) since only cross-platform frameworks are used.