Merge bisect-runner.pl fixes for 5.11-era parallel make bugs.

Merge bisect-runner.pl fixes for 5.11-era parallel make bugs.

"all known" - that's a description to tempt fate. But bisect-runner.pl
should now avoid falling foul of make race conditions introduced from late
December 2008, and only resolved in January 2010.

Without the fixes, the parallel make bugs could mean that a bisect would
sometimes fail to build any revision in the range. The best case would be a
"skip" on an intermediate revision, and a slightly slower bisect run. The
worst case would be the bisect reporting failure to complete, giving a
range of skipped revisions instead of a definitive answer.