Ideally, I think the solution would be to compile _math.c once, as a dependency shared between both extension modules (perhaps like a static library if necessary). Does anyone know if the build system supports this, or how to do it?

Another semi-favourable solution might be to compile _math.c twice, but send the output to separate directories, e.g. . . ./math-build/_math.o and . . ./cmath-build/_math.o. Again, I don’t know if this is practical.