Fix copy pasting with MSVC

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

Locale independence

This first prints the number into a temporary buffer and then copies the
number to the output.

A positive side effect is that cJSON no longer reserves more space for
the number in the output than is necessary.

Because sizeof('\0') is actually sizeof(int) not sizeof(char).

Found by coverity

Thanks @mmkeeper

This could only happen if the maximum SIZE_T is not at least 2 times
bigger than INT_MAX. Not sure if this can happen on real systems, but
better be safe then sorry.

The compiler flag detection was working incorrectly.