Add get_cutest_include_path function to system_paths

We really shouldn't have hardcoded include paths in `install_scripts.py`, this should be handled by `system_paths.py` like we already do with the CUTEst path on macOS otherwise it's just a mess.

The obvious way to do this is a `get_cutest_include_path` function in `system_paths.py` that looks for the `cutest.h` header.