Replace the call to int64_t in the blasutil test by explicit types

Some platforms define int64_t to be long long even for C++03. If this is
the case we miss the definition of internal::make_unsigned for this
type. If we just define the template we get duplicated definitions
errors for platforms defining int64_t as signed long for C++03.

We need to find a way to distinguish both cases at compile-time.