fuzz: Configure check for main function (take 2) #21080

Actually fix https://github.com/google/honggfuzz/issues/336#issuecomment-702972138

Follow-up to #20065

Steps to test: honggfuzz section in doc/fuzzing.md

On master:

• Incorrectly compiles with -DPROVIDE_MAIN_FUNCTION
• Thus, the persistent fuzz config fails:

# FUZZ=process_message_version  ./honggfuzz/honggfuzz -i ./qa-assets/fuzz_seed_corpus/process_message_version -- ./src/test/fuzz/fuzz 
[2021-02-04T13:19:41+0100][E][72620] cmdlineVerify():242 You must specify '___FILE___' if the -s (stdin fuzzing) or --persistent options are not set
[2021-02-04T13:19:41+0100][F][72620] main():325 Parsing of the cmd-line arguments failed

It is only possible to run the extremely slow -s (stdin) fuzz config:

# FUZZ=process_message_version  ./honggfuzz/honggfuzz -s -i ./qa-assets/fuzz_seed_corpus/process_message_version -- ./src/test/fuzz/fuzz
...
       Speed : 0/sec [avg: 13]
...

On this pull:

# FUZZ=process_message_version  ./honggfuzz/honggfuzz -i ./qa-assets/fuzz_seed_corpus/process_message_version -- ./src/test/fuzz/fuzz
...
       Speed : 13,184/sec [avg: 12,328]
...

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #20936 (build: build fuzz tests by default by danben)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

cr ACK fac4be30482c21ac330e09ef8756c49e37faa6fa: patch looks correct!

Thanks for fixing.