Fix int128 compatibility check(Issue #681)

[Dirreke]

Fix #681

delete 'config.define("HAVE_UINT128_EXTENSION", Some("1"));' in build.rs, which can cause Issue #681

Not all compilers support the type of int128, so the facebook/rocksdb#7338 uses makefile to check int128 compatibility. The type of int128 is not related to target, but related to compiler。

[stanislav-tkach]

I'm not sure about simply deleting it. It seems better to enable it conditionally.

[niklasf]

Definitely agree if there's a good way to feature-detect it. If not, deleting it should be fine. The gains (if there even are any) are pretty minor.

[JEnoch]

Definitely agree if there's a good way to feature-detect it

I think the CARGO_CFG_TARGET_POINTER_WIDTH environment variable could be used in build.rs :
https://doc.rust-lang.org/cargo/reference/environment-variables.html#environment-variables-cargo-sets-for-build-scripts

[niklasf]

I added this as part of #620 because it was easy to add, without testing if there are actually any advantages to this particular flag. But it turns out there is a clear disadvantage to adding it unconditionally, so I think we should just remove it, i.e., merge this PR.

If someone wants to bring it back later, they can figure out a way to feature-detect this.

[Dirreke]

Thanks!