Factor out the code that detects the long long int snprintf format into a

Factor out the code that detects the long long int snprintf format into a
separate macro.  Also add support for %I64d which is the way on Windows.

The code that checks for the 64-bit int type now gives more reasonable
results when cross-compiling: In that case we just take the compiler's
information and trust that the arithmetic works.  Disabling int64 is too
pessimistic.