[GTK] Add an optional moduleset with hard to get packages (including libsecret)

[GTK] Add an optional moduleset with hard to get packages (including libsecret)
https://bugs.webkit.org/show_bug.cgi?id=109195

Patch by Martin Robinson <mrobinson@igalia.com> on 2013-02-12
Reviewed by Philippe Normand.

Add an optional moduleset that includes libsecret. This moduleset will
be used to install some annoyingly hard to obtain dependencies on older
distributions.

* gtk/jhbuild-optional.modules: Added.
* gtk/jhbuild.modules: Add a reference to the new moduleset file.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@142620 268f45cc-cd09-0410-ab3c-d52691b4dbfc