call_matlab.h into dlib namespace, and also added missing overloads for
call_matlab().

MATLAB mex file.

Fixed a minor typo in the spec.

try_compile statements rather than a cuDNN specific variable.

Find cudnn fix

Support use of dlib a cross-compiled ExternalProject

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

automatic step size reduction code a little.

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