[CI][C++] test-build-cpp-fuzz failed because Boost libraries are required

[kou]

Describe the bug, including details regarding any error messages, version, and platform.

https://github.com/ursacomputing/crossbow/actions/runs/13125731106/job/36621579172#step:7:271

CMake Warning at /usr/local/share/cmake-3.29/Modules/FindBoost.cmake:1398 (message):
  New Boost version may have incorrect or missing dependencies and imported
  targets
Call Stack (most recent call first):
  /usr/local/share/cmake-3.29/Modules/FindBoost.cmake:1523 (_Boost_COMPONENT_DEPENDENCIES)
  /usr/local/share/cmake-3.29/Modules/FindBoost.cmake:2135 (_Boost_MISSING_DEPENDENCIES)
  cmake_modules/ThirdpartyToolchain.cmake:313 (find_package)
  cmake_modules/ThirdpartyToolchain.cmake:1292 (resolve_dependency)
  CMakeLists.txt:546 (include)

This happened after #45397 is merged.

In general, libarrow_testing.{a,so} that are also used by fuzzer require boost::filesystem because arrow::util::Process uses it:

arrow/cpp/src/arrow/testing/process.cc

Lines 76 to 92 in 1567be0

	# ifdef BOOST_PROCESS_USE_V2
	namespace process = BOOST_PROCESS_V2_NAMESPACE;
	namespace filesystem = process::filesystem;
	// For Boost < 1.87.0
	# ifdef BOOST_PROCESS_V2_ASIO_NAMESPACE
	namespace asio = BOOST_PROCESS_V2_ASIO_NAMESPACE;
	# else
	namespace asio = process::net;
	# endif
	# elif defined(BOOST_PROCESS_HAVE_V1)
	namespace process = boost::process::v1;
	namespace filesystem = boost::process::v1::filesystem;
	# else
	namespace process = boost::process;
	namespace filesystem = boost::filesystem;
	# endif
	#endif

But it seems that we can use libarrow_testing.a without boost::filesystem when we don't use arrow::util::Process.

test-build-cpp-fuzz prepare only Boost headers: https://github.com/google/oss-fuzz/blob/e007cb7be3868cb4f55e030c6c2a4199ca750bb6/projects/arrow/build.sh#L20-L28

So, let's require boost::filesystem only when libarrow_testing.so is used.

Component(s)

C++, Continuous Integration

[kou]

Issue resolved by pull request 45424
#45424