[C++] CMake Configuration Broken When Setting CMAKE_CXX_COMPILER=clang++

[WillAyd]

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

If you add -DCMAKE_CXX_COMPILER=clang++ you get a rather confusing error message about the threads library not being found:

$ cmake -S . -B build -DCMAKE_BUILD_TYPE=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS=ON -G Ninja -DARROW_COMPUTE=ON -DARROW_BUILD_TESTS=ON -DCMAKE_CXX_COMPILER=clang++
CMake Error at /usr/share/cmake-3.28/Modules/FindPackageHandleStandardArgs.cmake:230 (message):
  Could NOT find Threads (missing: Threads_FOUND)
Call Stack (most recent call first):
  /usr/share/cmake-3.28/Modules/FindPackageHandleStandardArgs.cmake:600 (_FPHSA_FAILURE_MESSAGE)
  /usr/share/cmake-3.28/Modules/FindThreads.cmake:226 (FIND_PACKAGE_HANDLE_STANDARD_ARGS)
  cmake_modules/ThirdpartyToolchain.cmake:1050 (find_package)
  CMakeLists.txt:546 (include)

If you look at the CMake configuration log, you will see that the error actually doesn't have anything to do with thread support, but rather with the C compiler (in my case gcc) being provided arguments that only work with clang:

---
events:
    kind: "try_compile-v1"
    backtrace:
      - "/usr/share/cmake-3.28/Modules/Internal/CheckSourceCompiles.cmake:101 (try_compile)"
      - "/usr/share/cmake-3.28/Modules/CheckCSourceCompiles.cmake:52 (cmake_check_source_compiles)"
      - "/usr/share/cmake-3.28/Modules/FindThreads.cmake:97 (CHECK_C_SOURCE_COMPILES)"
      - "/usr/share/cmake-3.28/Modules/FindThreads.cmake:163 (_threads_check_libc)"
      - "cmake_modules/ThirdpartyToolchain.cmake:1050 (find_package)"
      - "CMakeLists.txt:546 (include)"
    checks:
      - "Performing Test CMAKE_HAVE_LIBC_PTHREAD"
    directories:
      source: "/home/willayd/clones/arrow/cpp/build/CMakeFiles/CMakeScratch/TryCompile-C4sSBS"
      binary: "/home/willayd/clones/arrow/cpp/build/CMakeFiles/CMakeScratch/TryCompile-C4sSBS"
    cmakeVariables:
      CMAKE_CXX_COMPILER_CLANG_SCAN_DEPS: "/usr/bin/clang-scan-deps-18"
      CMAKE_C_FLAGS: "-O0 -O0 -Qunused-arguments"
      CMAKE_C_FLAGS_DEBUG: "-g -Werror -O0 -ggdb "
      CMAKE_EXE_LINKER_FLAGS: ""
      CMAKE_MODULE_PATH: "/home/willayd/clones/arrow/cpp/cmake_modules"
      CMAKE_POSITION_INDEPENDENT_CODE: "ON"
    buildResult:
      variable: "CMAKE_HAVE_LIBC_PTHREAD"
      cached: true
      stdout: |
        Change Dir: '/home/willayd/clones/arrow/cpp/build/CMakeFiles/CMakeScratch/TryCompile-C4sSBS'
        
        Run Build Command(s): /usr/bin/ninja -v cmTC_500bc
        [1/2] /usr/bin/cc -DCMAKE_HAVE_LIBC_PTHREAD  -O0 -O0 -Qunused-arguments  -std=gnu11 -fPIE -o CMakeFiles/cmTC_500bc.dir/src.c.o -c /home/willayd/clones/arrow/cpp/build/CMakeFiles/CMakeScratch/TryCompile-C4sSBS/src.c
        FAILED: CMakeFiles/cmTC_500bc.dir/src.c.o 
        /usr/bin/cc -DCMAKE_HAVE_LIBC_PTHREAD  -O0 -O0 -Qunused-arguments  -std=gnu11 -fPIE -o CMakeFiles/cmTC_500bc.dir/src.c.o -c /home/willayd/clones/arrow/cpp/build/CMakeFiles/CMakeScratch/TryCompile-C4sSBS/src.c
        cc: error: unrecognized command-line option ‘-Qunused-arguments’
        ninja: build stopped: subcommand failed.
        
      exitCode: 1

Component(s)

C++