* gc.c: is_incremental_marking(), will_be_incremental_marking():
  use `&&' with GC_ENABLE_INCREMENTAL_MARK intead of using
  #if/#else/#endif.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48157 b2dd03c8-39d4-4d8f-98ff-823fe69b080e