Add Debian/Ubuntu instructions to build Flight with system libs

cpp/README.md

@@ -339,6 +339,10 @@ When building, set the environment variables `GRPC_HOME` and/or
 
 On Ubuntu/Debian, you can install these dependencies with:
 
+```shell
+sudo apt-get install libgrpc-dev libgrpc++-dev protobuf-compiler-grpc libc-ares-dev
+```
+
 On macOS, you can use [Homebrew][1]:
 
 ```shell

cpp/cmake_modules/FindgRPC.cmake

@@ -47,8 +47,10 @@ else()
     NO_DEFAULT_PATH
     PATH_SUFFIXES ${lib_dirs})
 
+  # On Debian/Ubuntu, libaddress_sorting is statically linked.
   find_library (GRPC_ADDRESS_SORTING_STATIC_LIB
     NAMES "${CMAKE_STATIC_LIBRARY_PREFIX}address_sorting${CMAKE_STATIC_LIBRARY_SUFFIX}"
+          "${CMAKE_STATIC_LIBRARY_PREFIX}grpc++${CMAKE_STATIC_LIBRARY_SUFFIX}"
     PATHS ${_grpc_path}
     NO_DEFAULT_PATH
     PATH_SUFFIXES ${lib_dirs})

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1257,15 +1257,6 @@ if (ARROW_FLIGHT)
     # XXX(wesm): relying on vendored c-ares provided by gRPC for the time being
     set(CARES_STATIC_LIB "${GRPC_STATIC_LIBRARY_CARES}")
     set(GRPC_CPP_PLUGIN "${GRPC_BUILD_DIR}/${CMAKE_CFG_INTDIR}/grpc_cpp_plugin")
-    # get_property(GPR_STATIC_LIB TARGET gRPC::gpr PROPERTY LOCATION)
-    # get_property(GRPC_STATIC_LIB TARGET gRPC::grpc_unsecure PROPERTY LOCATION)
-    # get_property(GRPCPP_STATIC_LIB TARGET gRPC::grpc++_unsecure PROPERTY LOCATION)
-    # get_property(GRPC_ADDRESS_SORTING_STATIC_LIB
-    #   TARGET gRPC::address_sorting PROPERTY LOCATION)
-    # # XXX(wesm): relying on vendored c-ares provided by gRPC for the time being
-    # get_property(CARES_STATIC_LIB TARGET c-ares::cares_static PROPERTY LOCATION)
-    # # Get location of grpc_cpp_plugin so we can pass it to protoc
-    # get_property(GRPC_CPP_PLUGIN TARGET gRPC::grpc_cpp_plugin PROPERTY LOCATION)
   else()
     find_package(gRPC REQUIRED)
     set(GRPC_VENDORED 0)