Feature: <checked-iterators> 'on' --> _SECURE_SCL=1 'off' (default) --> _SECURE_SCL=0
I believe that the default should match the VC default, which is 1. This has no performance implications. Whether _SECURE_SCL is 0 or 1 in a particular project is decided by the user, not us. The autolink code will check the macro value and attempt to link to the appropriate Boost library. What is _built_ by default should match what is _used_ by default; it doesn't matter which one is faster, the choice is not ours to make.
Name mangling: <checked-iterators> adds 'c' to library name if enabled (assuming it's applicable).
Feature: <debugging-iterators> 'on' --> _HAS_ITERATOR_DEBUGGING defined 'off' (default) --> _HAS_ITERATOR_DEBUGGING not defined
Not defined means "on" in debug builds, "off" in release builds. Defined to 0 means off. Defined to 1 means "on" in debug builds, "off", warning and redefintion back to 0 in release builds. If we build no debug libraries, this default doesn't matter for MSVC. If we do, the default should again match the compiler default, either 1 or not defined.