[CI][C++] Flight feature verification failure

[pitrou]

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

Flight feature verification fails in the test-debian-11-python-3 CI job for reasons that have nothing to do with the feature being present/absent:

-- Checking support for TlsCredentialsOptions (gRPC >= 1.27)...
-- TlsCredentialsOptions (for gRPC 1.27) not found in grpc::experimental.
CMake Warning at src/arrow/flight/CMakeLists.txt:132 (message):
  Build output:
Call Stack (most recent call first):
  src/arrow/flight/CMakeLists.txt:154 (test_grpc_version)


CMake Warning at src/arrow/flight/CMakeLists.txt:134 (message):
  Change Dir: /build/cpp/src/arrow/flight/try_compile/CMakeFiles/CMakeTmp

  

  Run Build Command(s):/usr/bin/ninja cmTC_e1328 && [1/2] Building CXX object
  CMakeFiles/cmTC_e1328.dir/check_tls_opts_127.cc.o

  FAILED: CMakeFiles/cmTC_e1328.dir/check_tls_opts_127.cc.o

  /usr/lib/ccache/g++ -I/build/cpp/src -I/arrow/cpp/src
  -I/arrow/cpp/src/generated -Wno-noexcept-type -fdiagnostics-color=always
  -Wall -Wno-conversion -Wno-sign-conversion -Wunused-result -Wdate-time
  -fno-semantic-interposition -msse4.2 -g -Werror -O0 -ggdb -fPIE -pthread
  -std=c++11 -o CMakeFiles/cmTC_e1328.dir/check_tls_opts_127.cc.o -c
  /arrow/cpp/src/arrow/flight/try_compile/check_tls_opts_127.cc

  /arrow/cpp/src/arrow/flight/try_compile/check_tls_opts_127.cc: In function
  'int main(int, const char**)':

  /arrow/cpp/src/arrow/flight/try_compile/check_tls_opts_127.cc:34:39: error:
  unused variable 'opt' [-Werror=unused-variable]

     34 |   grpc_tls_server_verification_option opt = check(nullptr)
        |                                       ^~~

  cc1plus: all warnings being treated as errors

  ninja: build stopped: subcommand failed.

Component(s)

C++, Continuous Integration, Python