Remove all MSVC workarounds from global namespace

 * Fix how the problem is detected: checking __STDC_VERSION__ always
   fails because a C++ compiler never defines that even if the C99
   library is available. So fall back to detecting specific "broken"
   compilers like MSVC++ <= 9 (compiler version <= 15.00)
 * Prepare all code for addition of more "broken" compilers via extra
   #elif statements
 * Add a typedef picojson::int64_t (instead of the suggested int_t)
   because the name is more descriptive and people are by now used to
   int<width>_t.