for burnslib\core

USE_MSVCRT=1

links to dll version of CRTs.  If STL headers are included, also causes
link to msvcrp60.dll
causes -D_DLL=1 -D_MT=1 to be passed to cl, otherwise the same.

USE_LIBCMT=1

causes -D_MT=1 but not -D_DLL=1 to be passed to cl

_MT is necessary to get correct declaration of errno from stdlib.h
for binaries, causes link to thread-aware msvcrt.dll (non-static crts)