thread_pool's destructor.  The previous implementation used dlib's global
thread pooling to allocate threads to dlib::thread_pool, however, this
sometimes caused annoying behavior when used as part of a MATLAB mex file.

already enabled.  This also makes the global destruction order a little more
deterministic.

early stopping based on observing the loss on held out data.

that really needs it off.

* cmake script to suppress randlib warnings about *.a having no symbols on
MacOSX

* Moved script code into the main CMakeLists.txt file to suppress the
superfluous ranlib warnings all the time.

(in particular on mac os x, where this flag needs to be set this way for it to
work in some cases).

* Fixed issue with uninitialised variables. There are 2 places where std::exception_ptr eptr is not initialised.

* running_gradient.h needs to qualify erfc with 'std::' to avoid compilation error. 
Found when compiling dnn_mmod_face_detection_ex.cpp with RadStudio and Clang compiler.

distutils environment variables for all versions of python, not just conda.

is because some build systems seem to mess up the parallel build of these two
things and sometimes rebuild and potentially clobber object files during the
parallel build.  I haven't seen this result in a problem, but the race
condition does seem to exist in some build systems.  So this should force the
two top level targets to be build sequentially rather than in parallel, thus
avoiding any possible race condition.

arbitrary matlab objects.

doesn't work correctly in visual studio since visual studio doesn't define __cplusplus
to 201103 even though the current version ostensibly supports C++11.

Fix #363 by not setting CUDA_HOST_COMPILER when targeting Visual Studio