Support ruby 3.0+.

Index: extconf.rb
--- extconf.rb.orig
+++ extconf.rb
@@ -23,7 +23,7 @@ makefile += "OBJS=       $(SRCS:.c=.o)\n\n"
 
 makefile += ".c.o:\n"
 
-if RUBY_VERSION =~ /1.9/ then
+if RUBY_VERSION !~ /1.8/ then
     makefile += "	$(CC) $(CFLAGS) -c $< -I #{$topdir} -I #{$topdir}/#{RUBY_PLATFORM} -DRUBY_19\n\n"
 else
     makefile += "	$(CC) $(CFLAGS) -c $< -I #{$topdir} \n\n"
