ARROW-7838: [C++] Only link Boost libraries with tests, not libarrow.so

Enabling S3 or Flight tests would add a spurious link dependency between
libarrow (and therefore plasma-store-server) and Boost.

Closes #6438 from pitrou/ARROW-7838-boost-linking-plasma and squashes the following commits:

fb9c3ad <Antoine Pitrou> ARROW-7838:  Only link Boost libraries with tests, not libarrow.so

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/CMakeLists.txt

@@ -693,14 +693,7 @@ set(ARROW_SHARED_PRIVATE_LINK_LIBS ${ARROW_STATIC_LINK_LIBS})
 
 # boost::filesystem is needed for S3 and Flight tests as a boost::process dependency.
 if(((ARROW_FLIGHT OR ARROW_S3) AND (ARROW_BUILD_TESTS OR ARROW_BUILD_INTEGRATION)))
-  set(ARROW_WITH_BOOST_FILESYSTEM ON)
-
-  list(APPEND ARROW_SHARED_PRIVATE_LINK_LIBS ${BOOST_FILESYSTEM_LIBRARY}
-              ${BOOST_SYSTEM_LIBRARY})
-
-  list(APPEND ARROW_STATIC_LINK_LIBS ${BOOST_FILESYSTEM_LIBRARY} ${BOOST_SYSTEM_LIBRARY})
-
-  list(APPEND ARROW_STATIC_INSTALL_INTERFACE_LIBS boost_filesystem boost_system)
+  list(APPEND ARROW_TEST_LINK_LIBS ${BOOST_FILESYSTEM_LIBRARY} ${BOOST_SYSTEM_LIBRARY})
 endif()
 
 if(NOT MSVC)

cpp/src/arrow/CMakeLists.txt

@@ -199,10 +199,6 @@ set(ARROW_SRCS
 # Disable DLL exports in vendored uriparser library
 add_definitions(-DURI_STATIC_BUILD)
 
-if(ARROW_WITH_BOOST_FILESYSTEM)
-  add_definitions(-DARROW_WITH_BOOST_FILESYSTEM)
-endif()
-
 if(ARROW_WITH_BROTLI)
   add_definitions(-DARROW_WITH_BROTLI)
   list(APPEND ARROW_SRCS util/compression_brotli.cc)