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ARROW-7370: [C++] Fix old Protobuf with AUTO detection failure

If old Protobuf is found, protobuf::libprotobuf CMake target is
defined. The found old Protobuf isn't used and build vendored
Protobuf. The build step tries creating protobuf::libprotobuf CMake
target. But the target is already defined by the found old Protobuf.

This change introduces ARROW_PROTOBUF_* variables and uses ${them}
instead of raw protobuf::* targets to solve this problem.

Error message:

    -- Could NOT find Protobuf: Found unsuitable version "3.6.1", but required is at least "3.7.0" (found /usr/lib/x86_64-linux-gnu/libprotobuf.so;-pthread)
    Building Protocol Buffers from source
    CMake Error at cmake_modules/ThirdpartyToolchain.cmake:1179 (add_library):
      add_library cannot create imported target "protobuf::libprotobuf" because
      another target with the same name already exists.
    Call Stack (most recent call first):
      cmake_modules/ThirdpartyToolchain.cmake:147 (build_protobuf)
      cmake_modules/ThirdpartyToolchain.cmake:178 (build_dependency)
      cmake_modules/ThirdpartyToolchain.cmake:1204 (resolve_dependency_with_version)
      CMakeLists.txt:428 (include)

Closes #6011 from kou/cpp-protobuf-fix-fallback-build-failure and squashes the following commits:

52fa7964a <Sutou Kouhei>  Fix old Protobuf with AUTO detection failure

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Wes McKinney <wesm+git@apache.org>