Try more places for validation data; workaround CMake FindCUDA bug. (#233)

Fixes #232.

* Try `NMC_DATADIR` environment variable for validation data path, or else if the #defined `NMC_DATADIR` does not point to a directory, try `./validation/data` and `../validation/data`.
* Don't define `NMC_DATADIR` if CMake version 3.7 or 3.8.
* Extend C++17 filesystem emulation with (POSIX) implementation of `is_directory` and supporting classes, functions and enums `filesystem_error`, `file_status`, `status(..)`, `file_type` and `perms`.