For my programming language I have recently developed its own build system. I came to the conclusion, that introducing a separate configuration language isn't a good idea, since one need to learn it. So I have decided to use the language itself for its project description files.
A project description file is just a small program containing a single function, which returns project description in declarative form. The build system executable just compiles this program into a shared library, dynamically loads it and calls it to obtain project info. Then it executes the build, managing stuff like dependencies and incremental building.