Detect nightly in icu_capi_freertos, bump to 1.2.1

[Manishearth]

Alternative to #3382

This fixes the nightly breakage by introducing --cfg needs_alloc_error_handler and setting it via build script using the rustc_version crate.

I'm okay adding this dependency for now because FreeRTOS is a rare use case and it is likely that people who care about precise dep count will also be using a custom build system (where this build script won't run in the first place)

The polarity of this check is that recent nightly needs no cfg, since as I've mentioned in chat it is good practice when using Rust nightly to focus on recent nightly; and our happy path should be aligned with that.

[jira-pull-request-webhook]

Notice: the branch changed across the force-push!

• ffi/freertos/build.rs is now changed in the branch
• ffi/freertos/src/lib.rs is different

View Diff Across Force-Push

~ Your Friendly Jira-GitHub PR Checker Bot

[Manishearth]

Merging now so we have it for our nightly CI run in an hour. I'll do the publish in a while

[sffc]

lgtm, thanks!