bootstrap: Avoid redundant compiler selection checks
In commit 6e613ff3 (bootstrap: Add infrastructure to detect threading flags, 2017-11-28, v3.11.0-rc1~281^2~1) an extra level of nesting was added to the selection loop, but the inner-most `break` command used to exit the loop on success was not updated. This caused the outer-most loop to iterate unnecessarily and repeatedly try the same compilers again. In the case of compilers requiring a `-std=` flag, this may have caused the oldest standard to be used instead of the newest.
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