Granted, I'm not as familiar as some to the specific needs of writing a build system, but could you elaborate further on specifically what you want and specifically why it would be an improvement? Just adding constexpr to filesystem operations seems like it has a large area of confusion for the obvious reason of there being no guarantee that a program will be compiled and run on the same filesystem (as well as a few other reasons of varying obviousness).

Are you looking for a facility like #embed, to directly embed file data into your source code? Or are you thinking of the possibility of more complex file manipulation performed by the compiler?