* gc.c: guard by #if/#endif with GC_ENABLE_INCREMENTAL_MARK
  to hide unused codes.
* gc.c: similar to GC_ENABLE_LAZY_SWEEP.



git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48419 b2dd03c8-39d4-4d8f-98ff-823fe69b080e