Replace add_definitions with CMAKE_CXX_FLAGS in cmake file

When using add_definitions erroneous flags are passed to dll resource compiler under MinGW. This breaks the build for anything linked to dlib, using a dll rc and MinGW.

The proper way is to append to CMAKE_CXX_FLAGS.

For CMake 2.8.12 and higher the proper way is add_compile_options(-Wreturn-type) OR target_compile_options(dlib PRIVATE -Wreturn-type)

totally removing it, and also made --stats print the number of non-ignored
boxes.

really small boxes.

always going to be required for imglab.

non-square matrix.

out by Yan Xu.  One of the face locations was mistakenly used twice while
another was skipped.  This change breaks backwards compatibility with the
previous feature extraction output but should slightly improve accuracy of
classifiers trained using these features.

switching to projected gradient steps.

Revise help message

and try to link against that one rather than whatever might be installed
system wide.