functions.

Made LIB_INSTALL_DIR only appear when building dlib as an installable library, not when using dlib in another cmake project.

for each layer if you have passed a tensor through the next.

interface.

progress" estimate.  I also renamed the get/set functions for the shrink amount
to have a consistent name and use the word "factor" instead of "amount".

object as an input.  This allows the solvers to exhibit a more complex behavior
that depends on the specific layer.  It also removes the learning rate from the
solver's parameter set and pushes it entirely into the core training code.
This also removes the need for the separate "step size" which previously was
multiplied with the output of the solvers.

Most of the code is still the same, and in the core and trainer the step_size
variables have just been renamed to learning_rate.  The dnn_trainer's relevant
member functions have also been renamed.

The examples have been updated to reflect these API changes.  I also cleaned up
the resnet definition and added better downsampling.

skip layers and add_prev style layers.  In particular, now in-place layers only
overwrite the gradient information in their child layer if they are operating
in in-place mode.  Otherwise, they add their gradients to their child layers.

It should also be noted that it's safe for in-place layers to overwrite
gradients when in in-place mode since their child layers are inaccessible when
in-place layers operate in in-place mode.  This prevents any other layers from
trying to add to the child layer, thereby avoiding the potability of layer
interference.  So the bug this change fixes is that, when not in in-place mode
the child layers are still accessible but in-place layers were *still*
overwriting child gradients.

if so.  This way, if you have a long running mex file it will be killable if
it is periodically printing.

using the same seed.

ago.  This fixes that and turns them back into max pooling layers.

without getting a compiler error.

Augment build-system, allow for multilib installation, support pkg-config discovery

* Define LIB_INSTALL_DIR cache variable, allowing for multilib installations
* Discover BLAS and LAPACK via pkg-config if possible
* Fix incorrect toolchain variables in "dlib/test/makefile"

what it's contract says needs to happen (and what existing code expects).

non-one based strides.

interfaces.  Also changed the default behavior when the stride isn't 1.  Now
the filters will be applied only to the "valid" part of the image.

to expose it in the final layer interface.