introduce a configurable threshold for rendering HOGs

Fixed a minor typo in the spec.

Support use of dlib a cross-compiled ExternalProject

the former works with a wider range of tools in the STL.

This follows convention which simplifies using dlib as a CMake
ExternalProject.

since sometimes gfortran isn't available and openblas should already be linked
to gfortran if it was needed.

orthogonalize does not work with fixed-sized matrices

detect OS X, and query sysconfig for libdir

global functions.

CMake.