Disable march=native for SSE2

[bmwiedemann]

Without this patch, the code would only build binaries with SSE2,
if the build machine CPU supported SSE2.

And through the -march=native flag, it would then use more CPU features
available on the build CPU that might not be available
on the target machine CPU. That can cause an illegal instruction
exception on the target machine.

This patch changes behaviour so that only configure options determine the
result of the build, regarding SSE2.

See https://reproducible-builds.org/ for why this is good.

This PR was done while working on reproducible builds for openSUSE.

note that there are similar issues around AVX and neon extensions, but they are more tricky to resolve. e.g. the AVX test needs -avx2 for some reason and maybe avx512 should be decoupled.
Maybe an option for compile-time detection is wanted there by some users?

[genivia-inc]

Thank you for your contribution. This is important and useful.

Looks good for SSE2.

Frankly, I faced trouble getting the right sets of extended instructions to be recognized on some combinations of platforms with the GCC and clang compilers. Indeed, I don't like using -march=native but in some cases it would only appear to build properly with -march=native for AVX, even though it should have worked without this option according to the compiler documentation.

[bmwiedemann]

I must admit that I only tested this on openSUSE Tumbleweed with gcc10. Hope it still works out with other setups.