#error "Don't put the dlib folder in your include path"
You are getting this error because you have added the dlib folder to your
compiler's include search path.
You should *NOT* add the dlib folder itself to your compiler's include path.
Doing so will cause the build to fail because of name collisions (such as
dlib/string.h and string.h from the standard library). Instead you should
add the folder that contains the dlib folder to your include search path
and then use include statements of the form #include <dlib/queue.h> or
#include "dlib/queue.h". This will ensure that everything builds correctly.
The XCode IDE often puts all folders that it knows about into
the compiler search path. So if you are using XCode then either
don't drag the whole dlib folder into the project or alternatively
modify your XCode project settings to not auto-add all folders to
the include path. Instead just make sure that the dlib folder is
itself inside a folder in your include path.