Don't echo empty string. (Remove workaround for windows)

mrbgems/mruby-bin-mruby-config/mrbgem.rake

@@ -23,7 +23,7 @@ MRuby.each_target do
     FileUtils.copy "#{File.dirname(__FILE__)}/#{mruby_config}", t.name
     config = Hash[open("#{build_dir}/lib/libmruby.flags.mak").read.split("\n").map {|x| x.split(/\s*=\s*/, 2)}]
     IO.write(t.name, File.open(t.name) {|f|
-      f.read.gsub (/echo (MRUBY_CFLAGS|MRUBY_LDFLAGS|MRUBY_LIBS)/) {|x| config[$1].empty? ? 'echo/' : "echo #{config[$1]}"}
+      f.read.gsub (/echo (MRUBY_CFLAGS|MRUBY_LDFLAGS|MRUBY_LIBS)/) {|x| config[$1].empty? ? '' : "echo #{config[$1]}"}
     })
     FileUtils.chmod(0755, t.name)
   end