Bug #225288 --> turn on the pref extension for thunderbird.

Effects thunderbird build only.

configure.in

@@ -3630,7 +3630,7 @@ mail)
   MOZ_STATIC_MAIL_BUILD=1
   MOZ_COMPOSER=1
   MOZ_APP_VERSION=`cat $topsrcdir/mail/config/version.txt`
-  MOZ_EXTENSIONS_DEFAULT=" wallet spellcheck xmlextras webservices"
+  MOZ_EXTENSIONS_DEFAULT=" wallet spellcheck xmlextras pref webservices"
   AC_DEFINE(MOZ_THUNDERBIRD)
   ;;