You could try putting TGUI in a folder which does not contain spaces (e.g. "D:/SFML/TGUI-Build/") or putting it on the C drive to see if it changes anything.
If that doesn't work then it looks like either the compiler or cmake is broken.
All commits that I push to github are automatically tested on different platforms so it is unlikely that there is a fix that I can make, it looks like something specific to your pc.