[ruby/stringio] Make STRINGIO_VERSION uniform

https://github.com/ruby/stringio/commit/4400bf3380

ext/stringio/stringio.c

@@ -12,7 +12,8 @@
 
 **********************************************************************/
 
-#define STRINGIO_VERSION "3.0.9"
+static const char *const
+STRINGIO_VERSION = "3.0.9";
 
 #include "ruby.h"
 #include "ruby/io.h"

ext/stringio/stringio.gemspec

@@ -4,7 +4,7 @@
 source_version = ["", "ext/stringio/"].find do |dir|
   begin
     break File.open(File.join(__dir__, "#{dir}stringio.c")) {|f|
-      f.gets("\n#define STRINGIO_VERSION ")
+      f.gets("\nSTRINGIO_VERSION ")
       f.gets[/\s*"(.+)"/, 1]
     }
   rescue Errno::ENOENT