Updated afl to 2.39b.

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Version 2.39b:
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  - Improved error reporting in afl-cmin. Suggested by floyd.

  - Made a minor tweak to trace-pc-guard support. Suggested by kcc.

  - Added a mention of afl-monitor.

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Version 2.38b:
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  - Added -mllvm -sanitizer-coverage-block-threshold=0 to trace-pc-guard
    mode, as suggested by Kostya Serebryany.

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Version 2.37b:
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  - Fixed a typo. Spotted by Jakub Wilk.

  - Fixed support for make install when using trace-pc. Spotted by
    Kurt Roeckx.

  - Switched trace-pc to trace-pc-guard, which should be considerably
    faster and is less quirky. Kudos to Konstantin Serebryany (and sorry
    for dragging my feet).

    Note that for some reason, this mode doesn't perform as well as
    "vanilla" afl-clang-fast / afl-clang.


(wiz)