Add special build mode for OSS-Fuzz

[kevinbackhouse]

This is a follow-up to #1869. It turns out that even the -fsanitize=fuzzer-no-link and -fsanitize=fuzzer flags don't work with the OSS-Fuzz build. Instead, we have to add $LIB_FUZZING_ENGINE as a linker flag when we build the fuzz target. That's because OSS-Fuzz also wants to build for AFL and honggfuzz, not just libFuzzer. Each of those fuzzers uses a different main function wrapper around LLVMFuzzerTestOneInput.

[codecov]

Codecov Report

Merging #1875 (cee9015) into main (f8bfad1) will not change coverage.
The diff coverage is n/a.

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  Coverage   60.77%   60.77%           
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  Files          96       96           
  Lines       18887    18887           
  Branches     9498     9498           
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  Hits        11478    11478           
  Misses       5116     5116           
  Partials     2293     2293           

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[clanmills]

This looks good, @kevinbackhouse

My only request is to document these options in README.md.

[kevinbackhouse]

@clanmills: Thanks! I'll update the documentation in a follow-up PR. I want to include hyperlinks to the build script in the OSS-Fuzz repo, so it will be easier if I wait until they have merged my pull request.

[clanmills]

That's fine, @kevinbackhouse I approved it so you could push on with the OSS-Fuzz business. Somebody else could/should take on the documentation.