This should fix copy pasting of the C and header files with the MSVC
compiler.

This is necessary, because you can get it wrong if you do it manually.
(when you forget the & 0xFF in the comparison)

This assigns the macro cJSON_Invalid to 0.

This might cause slight changes in behavior, but it shouldn't break
anything that is not already broken (for example the original macro
could either return a double, or an integer or whatever depending on if
object is NULL or not.)

This is useful to programmatically find out the version of cJSON that
has been used (useful in case of scripting language bindings for
example).

This is important so that it is always known which version of the
library is used, especially if the C and Header files have just been
copy pasted to another code base.

Add a comment that warns about potential write access to constant memory
locations after using cJSON_AddItemToObjectCS.