Fix for srcdir with spaces

Fixes #22.
Build failure when the ruby installed directory name contains spaces.

ext/etc/extconf.rb

@@ -43,9 +43,15 @@
 
 # for https://github.com/ruby/etc
 srcdir = File.expand_path("..", __FILE__)
-if !File.exist?("#{srcdir}/depend")
-  %x[#{RbConfig.ruby} #{srcdir}/mkconstants.rb -o #{srcdir}/constdefs.h]
+constdefs = "#{srcdir}/constdefs.h"
+ruby = RbConfig.ruby
+if File.file?(ruby)
+  ruby = [ruby]
+else
+  require "shellwords"
+  ruby = Shellwords.split(ruby)
 end
+system(*ruby, "#{srcdir}/mkconstants.rb", "-o", constdefs)
 
 # TODO: remove when dropping 2.7 support, as exported since 3.0
 have_func('rb_deprecate_constant(Qnil, "None")')