Build.rs fails when building with -Z build-std

[ubamrein]

The autocfg probe function compiles sample code to the specified target to probe for i128 support. This fails when we try to use the crate with a target which does not have pre-built std-libraries and hence build it from scratch with the unstable build-std feature. I wonder if we could add an environment variable to just skip this check, or if there is any other way to check for i128 support (is it really only compiler version dependent? if so would a rustc --version be sufficient?).

I can gladly try to submit a pull-request, as I currently have the situation to test if the fix would work, but just wanted to ask for the right direction.

[dignifiedquire]

we probably can just drop the probe, as the MSRV is now high enough