dlib: 19.9 -> 19.13 (#41953)

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