Add compatibility headers for someC99 features.

nxn is mostly C89, with the following exceptions:

    1. use of stdint.h
    2. use of stdbool.h
    3. use of inttypes.h (actually, don't use anywhere at the moment,
            but goes hand-in-hand with stdint.h)
    4. variadic macros (only in NXN_DEBUG_LOG)

The files compat-stdint.h, compat-stdbool.h, compat-inttypes.h have been added;
at the moment, they are mostly for supporting Windows.

While this is a manageable set of C99 features to accept responsibility for, I
think it can be reduced even further (e.g., only a small set of stdint.h and
inttypes.h is actually needed; NXN_DEBUG_LOG should be changed to remove
varargs).