Enable compiler warnings in fuzztest targets

Pass the SYSTEM argument to add_subdirectory(ext/fuzztest) to treat
FuzzTest headers as system headers. This suppresses compiler warnings in
FuzzTest headers.

Fix #2350.

tests/CMakeLists.txt

@@ -184,10 +184,7 @@ if(AVIF_ENABLE_FUZZTEST)
  macro(add_avif_fuzztest TEST_NAME)
  add_executable(${TEST_NAME} gtest/${TEST_NAME}.cc gtest/avif_fuzztest_helpers.cc ${ARGN})
  # FuzzTest bundles GoogleTest so no need to link to gtest librairies.
- # NOTE: FuzzTest and Abseil headers have compiler warnings (mostly
- # -Wsign-compare and some -Wunused-parameter and -Wshorten-64-to-32),
- # so we don't enable compiler warnings on the fuzztest targets.
- target_link_libraries(${TEST_NAME} PRIVATE aviftest_helpers_internal)
+ target_link_libraries(${TEST_NAME} PRIVATE aviftest_helpers_internal avif_enable_warnings)
  link_fuzztest(${TEST_NAME})
  add_test(NAME ${TEST_NAME} COMMAND ${TEST_NAME})
  set_property(TEST ${TEST_NAME} PROPERTY ENVIRONMENT "TEST_DATA_DIRS=${CMAKE_CURRENT_SOURCE_DIR}/data/")
@@ -201,7 +198,11 @@ if(AVIF_ENABLE_FUZZTEST)
  # FUZZTEST_REPLAY=/path/to/repro_file.test
  # and running one of the targets below.
  # See https://github.com/google/fuzztest/blob/main/doc/quickstart-cmake.md
- add_subdirectory(${AVIF_SOURCE_DIR}/ext/fuzztest ${AVIF_SOURCE_DIR}/ext/fuzztest/build.libavif EXCLUDE_FROM_ALL)
+ # Note: The SYSTEM argument for add_subdirectory() is new in CMake
+ # version 3.25. There are compiler warnings in FuzzTest headers. Add
+ # the ext/fuzztest subdirectory with the SYSTEM directory property so
+ # that warnings in its headers are suppressed.
+ add_subdirectory(${AVIF_SOURCE_DIR}/ext/fuzztest ${AVIF_SOURCE_DIR}/ext/fuzztest/build.libavif EXCLUDE_FROM_ALL SYSTEM)
  else()
  message(
  FATAL_ERROR