Problem mixing PICOJSON_USE_INT64 with non-PICOJSON_USE_INT64

[radoslawkojdecki]

In large C++ projects, one may encounter the mixing of picojson.h compiled with and without the PICOJSON_USE_INT64 flag. This can lead to very difficult-to-debug problems. I have created an example illustrating this issue here: https://github.com/radoslawkojdecki/picojson-bug-example. It should be noted that mixing such compilations should be impossible. Consider implementing a 'semaphore' to prevent compilation in such cases.