Deprecation of C-only build

[scivision]

Since this project was started, optimized compilers (AOCC, oneAPI, NVHPC, Cray) common on workstations and HPC systems have greatly improved C++ support. The main issue historically was that <filesystem> was not available. C++17 string_view is now widely available across such compilers for some time.

We plan to require C++17 support, with a C++ fallback when <filesystem> is not available. Until now, the fallback has been to C99 code and using CWalk.

We desire to eliminate CWalk as we had to process input and output to get the desired result, and it was more efficient to simply code our own functions instead of CWalk.