Initially the expression was quite small (just inherited properties from
`pkgs.dlib`), but the more it grows the better it is to store it into
its own file.
This fixes the latest `dlib` build for `nixpkgs` compliancy:
* Patched `setup.py` to use number of jobs defined in `$NIX_BUILD_CORES`
rather than using all available cores.
* Bumped `dlib` to latest version (v19.13 ATM).
* Dropped `openblas` build input, `cblas` which actually works lives in
`dlib/external`. Otherwise the test suite runs into segfaults (see
https://github.com/NixOS/nixpkgs/pull/39255#issuecomment-384535129 for
further reference).
* Added myself as maintainer in case of any further breakage in the
future.
Closes #39255
/cc @dotlambda @ryantm
