run bootstrap automatically

if a system has an older version of autoconf installed, building the
gem's native extensions fail.  run bootstrap before configure so that
autoreconf is run and the configuration succeeds.

extconf.rb

@@ -47,4 +47,5 @@
   env = "ARCHFLAGS='-arch #{arch}' CPPFLAGS='-arch #{arch}'"
 end
 
+system "sh bootstrap"
 system "#{env} sh configure --with-ruby=#{with_ruby} --prefix=#{prefix_dir} #{other_opts}"