Well... If you just want to take old abandoned source and somehow run it - anything goes (IIRC, NumPy requires Fortran for MKL). I meant old projects that are still maintained at some level yet are large in scope and cannot be reimplemented incrementally, think OpenSSL.

If we are talking about e.g. servers, we can safely assume pretty beefy x86-compatible hardware and discuss in the context of that. In my book C is the ultimate at general-purposeness and anything we attempt to do with C must be discussed in that light.