Skip C comments from scan target; close #5072

The original PR was skipping Ruby comments as well, but caused some
issues in test suites.

Rakefile

@@ -163,6 +163,7 @@ op_table = {
 file presym_file => cfiles+rbfiles+psfiles+[__FILE__] do
   csymbols = cfiles.map do |f|
     src = File.read(f)
+    src.gsub!(/\/\/.+(\n|$)/, "\n")
     [src.scan(/intern_lit\([^\n"]*"([^\n "]*)"/),
      src.scan(/mrb_define_method\([^\n"]*"([^\n"]*)"/),
      src.scan(/mrb_define_class_method\([^\n"]*"([^\n"]*)"/),