Always include build_config.rb to compile dependency.

build_config.rb modifies `cc.defines` (eg. `DISABLE_GEMS`) so files depending on such flags should be rebuilt.

Ref. (Failing scenario): https://gist.github.com/cho45/9181191

tasks/mruby_build_commands.rake

@@ -110,7 +110,7 @@ module MRuby
         File.read(file).gsub("\\\n ", "").scan(/^\S+:\s+(.+)$/).flatten.map {|s| s.split(' ') }.flatten
       else
         []
-      end
+      end + [ MRUBY_CONFIG ]
     end
   end