Merge pull request #511 from ruby/pz-compact-check

Check for GC compaction support

Author: peterzhu2118

harness/harness-common.rb

@@ -8,7 +8,7 @@
 end
 
 # Support enabling GC auto-compaction via environment variable
-GC.auto_compact = !!ENV["RUBY_GC_AUTO_COMPACT"]
+GC.auto_compact = !!ENV["RUBY_GC_AUTO_COMPACT"] if GC.respond_to?(:auto_compact)
 
 # Seed the global random number generator for repeatability between runs
 Random.srand(1337)