auto merge of #17890 : pnkfelix/rust/fsk-fix-issue-17887, r=alexcrichton

Fixes `config.mk` so that it should not contain multiple inconsistent entries for the same option.

Used aforementioned variants to extract options that have explicit `putvar` calls associated with them in the subsequent code.  When the explicit `putvar` call was conditional on some potentially complex condition, moved the `putvar` call out to the main control flow of the script so that it always runs if necessary.

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As a driveby fix, captured the error exit when doing the test run of `rustc --version` from `CFG_LOCAL_RUST_ROOT`, and signal explicit configure failure when it did not run successfully.  (If we cannot run `rustc`, we really shouldn't try to keep going.)

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Fix #17887.