ARROW-6447: [C++] Allow rest of arrow_objlib to build in parallel while memory_pool.cc is waiting on jemalloc_ep

Using OBJECT_DEPENDS like this seems to only work with the Ninja build generator with CMake 3.12.x, which is probably a bug. With Makefiles we fall back to the old strategy of blocking on the completion of jemalloc_ep.

Closes #5281 from wesm/ARROW-6447 and squashes the following commits:

7e46ff4 <Wes McKinney> Only use OBJECT_DEPENDS with Ninja build generator
c93f311 <Wes McKinney> cmake-format
eb52965 <Wes McKinney> Add jemalloc_ep dependency only on memory_pool.cc

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>

Author: wesm

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1408,7 +1408,6 @@ if(ARROW_JEMALLOC)
                                    INTERFACE_INCLUDE_DIRECTORIES
                                    "${CMAKE_CURRENT_BINARY_DIR}/jemalloc_ep-prefix/src")
   add_dependencies(jemalloc::jemalloc jemalloc_ep)
-  add_dependencies(toolchain jemalloc_ep)
 endif()
 
 # ----------------------------------------------------------------------

cpp/src/arrow/CMakeLists.txt

@@ -145,6 +145,14 @@ set(ARROW_SRCS
     util/utf8.cc
     vendored/datetime/tz.cpp)
 
+if(ARROW_JEMALLOC)
+  if("${CMAKE_GENERATOR}" STREQUAL "Ninja")
+    set_source_files_properties(memory_pool.cc PROPERTIES OBJECT_DEPENDS jemalloc_ep)
+  else()
+    add_dependencies(arrow_dependencies jemalloc_ep)
+  endif()
+endif()
+
 if(ARROW_JSON)
   add_subdirectory(json)
   set(ARROW_SRCS
@@ -205,10 +213,6 @@ if(ARROW_CUDA)
   add_subdirectory(gpu)
 endif()
 
-if(ARROW_JEMALLOC AND JEMALLOC_VENDORED)
-  add_dependencies(arrow_dependencies jemalloc::jemalloc)
-endif()
-
 if(ARROW_WITH_BROTLI)
   add_definitions(-DARROW_WITH_BROTLI)
   set(ARROW_SRCS util/compression_brotli.cc ${ARROW_SRCS})