Avoid MSC16 warning C4482 (might consider disabling it)
Fix comment
Minor reorganization of early #defines/#includes
Group ABC_HOST_* #defines
Define _DEBUG whenever DEBUG is #defined
MSC16: use debug CRT DLL since we build with DEBUG=1
Suppress MSC16 warning C4548 for MSC16’s own malloc.h
Fix heap corruption from raw_vextr_transaction::_construct() prematurely updating m_pEnd on realloc