Consider enabling compiler warnings in add_avif_fuzztest() in tests/CMakeLists.txt

[wantehchang]

@jzern @vrabaud @y-guyon

In #2344 I enabled compiler warnings in the tests/ directory, but I had to exclude the add_avif_fuzztest() macro because there are compiler warnings in the FuzzTest and Abseil headers.

It would be good to look into whether it is possible to enable compiler warnings in add_avif_fuzztest(), e.g., by turning off the warnings in the FuzzTest and Abseil headers.

I am surprised that Abseil headers have compiler warnings when compiled with -Wall -Wextra. The Abseil Compiler Flags page says they compile with -Wall -Wextra under GCC but turn off -Wsign-compare (which is one of the warnings I saw). So we can first suppress the compiler warnings in Abseil headers under the guidance of the Abseil Compiler Flags page, and then see what we need to do with the compiler warnings in FuzzTest headers.

[vrabaud]

In CMake 3.25 and above, we can add the SYSTEM property to the add_subdirectory of the fuzztest folder. fuzztest only recommends to use an add_subdirectory but we could try to FetchContent it as imported targets are SYSTEM I believe.

[wantehchang]

We ignore the compiler warnings when FuzzTest itself is compiled. This issue is about the compiler warnings when our fuzztest source files are compiled (because they include FuzzTest headers).

[wantehchang]

Vincent: I misunderstood your comment. Sorry. (I didn't know that compilers do not warn about issues in system headers.) I think adding the SYSTEM property to the add_subdirectory() call of the fuzztest folder is a good solution. Thanks for the suggestion. I am testing it in #2356.