GH-47914: [C++] Fix system Apache ORC used detection

We need to use `orc_SOURCE` not `ORC_SOURCE`.

Author: kou

cpp/src/arrow/CMakeLists.txt

@@ -80,7 +80,7 @@ if(ARROW_WITH_ZSTD)
 endif()
 
 if(ARROW_ORC)
-  if(ORC_SOURCE STREQUAL "SYSTEM")
+  if(orc_SOURCE STREQUAL "SYSTEM")
     list(APPEND ARROW_STATIC_INSTALL_INTERFACE_LIBS orc::orc)
   endif()
 endif()