Fix the warnings when building with C++, add MSVC C++ build support. This also changes the internal type of lotags, hitags and cstat type fields to uint16_t instead of int16_t to clean up some dubious behavior wherein the code was using a value of 32768 as a flag in these fields for certain types of things, like invisible sprites (using the value as if it was uint16_t) where it was elsewhere checking if the value was < 0 (using the value as if it was int16_t). This change may break a few specific effects if any part of the relevant code was missed when looking for areas that needed to be addressed.
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I think there's also a fix for the CON precache system breakage in here (lost it in my local tree when I started getting the C++ build working in MSVC, sorry!)