[GTK] Desktop proxy settings are ignored inside the internal jhbuild

[GTK] Desktop proxy settings are ignored inside the internal jhbuild
https://bugs.webkit.org/show_bug.cgi?id=120833

Patch by Mario Sanchez Prada <mario.prada@samsung.com> on 2013-09-06
Reviewed by Gustavo Noronha Silva.

Add dconf as an optional module for the internal jhbuild, along
with vala 0.17 as a dependency, and make sure we do not explicitly
set always the 'memory' backend, so we can have an easy way to
allow apps access desktop settings from the jhbuild. This is
useful for example to be able to run apps such as MiniBrowser
inside the jhbuild (by means of run-launcher, for instance) while
working behind a proxy, since those network settings have to be
read from the system global configuration in org.gnome.system.proxy.

* gtk/jhbuild-optional.modules: Added dconf 0.14 and vala 0.17.
* gtk/jhbuildrc: Do not explicitly choose the GSettings memory
backend here, since that will make more difficult to use the dconf
backend from inside the jhbuild if we need to do it so.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@155197 268f45cc-cd09-0410-ab3c-d52691b4dbfc