I wonder if they considered build2[1]? It seems to tick most of their boxes. In particular, it has first-class Windows support and, in the upcoming release, even native Clang-targeting-MSVC support (not the clang-cl wrapper)[2]. While some of the features they are looking for (like faster build times which I assume means distributed compilation and caching) are not there yet, we are working on them (and would welcome help)[3].