/// Determine at compile time the best popcount<> specialization according to
/// whether the platform is 32 or 64 bit, the maximum number of non-zero
/// bits to count and if the hardware popcnt instruction is available.
/// Determine at compile time the best popcount<> specialization according to
/// whether the platform is 32 or 64 bit, the maximum number of non-zero
/// bits to count and if the hardware popcnt instruction is available.