Use `conf.enable_debug`; close #5095

According to `doc/guides/compile.md`; thanks to @YunzheZJU

target/host.rb

@@ -80,7 +80,7 @@ MRuby::Build.new do |conf|
   # conf.file_separator = '/'
 
   # Turn on `enable_debug` for better debugging
-  enable_debug
+  conf.enable_debug
   conf.enable_bintest
   conf.enable_test
 end