Don't autodetect SSE2

[tornaria]

Instead, test SSE2 in the compiler.

This doesn't make a difference on x86_64 as sse2 is always present.
To enable sse2 for i686, one can add -msse2 to CFLAGS.

This partly addresses #9. In the current version, simd flags would be leaked
from the build box, even unused ones as avx2 and avx512.

[malb]

This is two separate changes in one commit, right? I'm not challenging that, I'm just trying to make sure I follow what is being proposed here:

• don't detect SSE2
• don't leak SIMD flags

[tornaria]

This is two separate changes in one commit, right? I'm not challenging that, I'm just trying to make sure I follow what is being proposed here:

* don't detect SSE2

* don't leak SIMD flags

Yes. But it's really one change, not detecting sse2; and then there are no simd flags to leak.

Since m4ri only uses sse2, there's nothing to loose here (x86_64 includes sse2).

[malb]

Ta!