Merge pull request #3021 from zzak/fix-enable-debug-define

Fix MRB_ENABLE_DEBUG_HOOK config typo left from 4440566b

build_config.rb

@@ -98,7 +98,7 @@ MRuby::Build.new('host-debug') do |conf|
   conf.gembox 'default'
 
   # C compiler settings
-  conf.cc.defines = %w(MRB_ENABLE_DEBUG)
+  conf.cc.defines = %w(MRB_ENABLE_DEBUG_HOOK)
 
   # Generate mruby debugger command (require mruby-eval)
   conf.gem :core => "mruby-bin-debugger"