[src] Add CUDA accelerated MFCC computation. (#3348)

* Add CUDA accelerated MFCC computation.

Creates a new directory 'cudafeat' for placing cuda feature extraction
components as it is developed.  Added a directory 'cudafeatbin' for
placing binaries that are cuda accelerated that mirrior binaries
elsewhere.

This commit implements:
  feature-window-cuda.h/cu which implements a feature window on the device
    by copying it from a host feature window.
  feature-mfcc-cuda.h/cu which implements the cuda mfcc feature
    extractor.
  compute-mfcc-feats-cuda.cc which mirriors compute-mfcc-feats.cc

  There were also minor changes to other files.

* Only build cuda binaries if cuda is enabled