[C++][CMake] gRPC doesn't respect CMAKE_CXX_COMPILER

[asfimport]

I have both gcc and clang installed. gcc is the default compiler, /usr/bin/c++ points to g++.
When build flight with clang, it failed with below message.

$ cmake -GNinja -DCMAKE_BUILD_TYPE=Release -DARROW_FLIGHT=ON -DARROW_WITH_ZLIB=ON -DCMAKE_CXX_COMPILER=/usr/bin/clang++-9 -DCMAKE_C_COMPILER=/usr/bin/clang-9 ..

$ ninja

    /usr/bin/c++    -Qunused-arguments -fcolor-diagnostics -O3 -DNDEBUG -O3 -DNDEBUG -fPIC -o CMakeFiles/cmTC_0ad35.dir/testCXXCompiler.cxx.o -c testCXXCompiler.cxx
    c++: error: unrecognized command line option ‘-Qunused-arguments’; did you mean ‘-Wunused-parameter’?
    c++: error: unrecognized command line option ‘-fcolor-diagnostics’

The reason is grpc/absl/cares building steps don't respect CMAKE_CXX_COMPILER and CMAKE_C_COMPILER.
They're always built with system default compiler.

Reporter: Yibo Cai / @cyb70289
Assignee: Yibo Cai / @cyb70289

PRs and other links:

• GitHub Pull Request #9002

_{Note: This issue was originally created as ARROW-11023. Please see the migration documentation for further details.}

[asfimport]

Kouhei Sutou / @kou:
Issue resolved by pull request 9002
#9002