AVX2 optimized version of get_checksum1() for x86-64

AVX2 optimized version of get_checksum1() for x86-64

Additionally restructures build switches and defines from SSE2 to SIMD,
to allow potential reuse should patches become available with SIMD
instructions for other processor architectures.

(Some minor tweaks of Jorrit's patch to avoid requiring GNU make and to
avoid C++ comments in .c files.)