fixes apalmblad#26; maintains backwards compatibility

Author: cgoettel

extconf.rb

@@ -10,6 +10,7 @@
 $CFLAGS = case RUBY_VERSION
           when /^1\.9/; '-DRUBY19'
           when /^2\./; '-DRUBY19'
+          when /^3\./; '-DRUBY19'
           else; ''
           end