partial C++11 support and funny cmake behavior of clang on OS X.

to new versions of clang on OSX, without this extra statement C++11 wasn't
being enabled.

map_output_to_input() in the concat_ layer.

Add MKL DFTI FFT bindings.

Apparently, MKL installations don't always include header files. So
detect MKL libraries and headers seperately.

feature_addition : Added a mean squared loss layer to DNN with test case

MKL FFT bindings had assumed that MKL was also being used to provide the
BLAS implementation, however the build system allows these 2 things to
be configured independantly.

Added mean squared loss layer "loss_mean_squared" to
DNN as requested in https://github.com/davisking/dlib/issues/152
Also added test case of a simple linear regression with one variable
that uses this layer.

unicode: Disable misleading-indentation warnings with GCC pragma

increasing loss more robust.  Now it will never get into a situation where it
backtracks over and over.  Instead, it will only backtrack a few times in a row
before just letting SGD run unimpeded.

reachable.