How to build update source code

[utopia]

Under https://freefilesync.org/download.php I could get the FreeFileSync 8.5 Source.
But to work with the Source there are some major Problems:
* there is no documentation how to build from Source Code
* there is no (git-) repository for Version handling
* there is no high Level documentation of the source Code

So my question:
* Does the author want, that others use the source and maybe contribute to it?
(I could understand, when he only want to handle his own code)
* What about a feature-fork for some special purpose under gpl

By the way:
is there a portable Version of 8.5?