Added support for a WITH_CARES option to be used when invoking nmake
via Makefile.vc. This option enables linking against both the DLL and
static versions of the c-ares libraries, as well as the debug and
release varients, depending on the value of DEBUG. The USE_ARES
preprocessor symbol is also defined.
This is a separate makefile for MSVC builds. It is deliberately put in
another dir than src/ and lib/ to allow a different build experience
than the previous - at least during a period. Eventually we should
unify.
