Upgrade to libxml2 2.9.1

https://bugzilla.gnome.org/show_bug.cgi?id=686118 was fixed in 2.9.1,
so there is no need for that patch anymore.
Build bison by default

Recent versions of GStreamer require bison 2.4, and OSX has only 2.3 as
of Mavericks.
Update download address for faad2

Sourceforge seems to have changed around their download URLs.
Build liboil from tarball

liboil is in maintenance mode and will not receive any more updates.
Since we need to patch it because AM_CONFIG_HEADER won't build any more
with Automake, we should build it from tarballs going forward.
Add missing liborc to modulesets

gstreamer-1.0 depended on it, but it had previously escaped addition.
Standardize meta-gstreamer modules

The metamodules are now all named meta-gstreamer[-1.0], not
meta-gtk-osx-gstreamer, and they do not pull in meta-gtk-osx-core nor
liboil/liborc (which is already pulled in by gstreamer.)
Remove spurious -- from autogenargs
Rename gst-ffmpeg-1.0 to gst-libav

The gst-ffmpeg module was renamed to gst-libav when GStreamer moved to
the 1.0 API, so let's reflect that naming in our module names.
Fix dependencies

In a few instances, gstreamer-1.0 modules depended on other
gstreamer-0.10 modules.
Fix typo in gst-plugins-ugly
Give gstreamer-1.0 modules separate checkoutdirs

This is so that you can build both gstreamer 0.10 and 1.0 APIs in
parallel.
Upgrade GStreamer versions

This upgrades all GStreamer modules in modulesets-stable to the latest
releases on the stable branches, and all in modulesets to the latest
stable branches.
Split unstable GStreamer into 0.10 & 1.0

This moves all previous GStreamer modules in modulesets-unstable to the
0.10 branch, and adds new 1.0 modules, and a new 1.0 metamodule, so that
all the packages and metamodules refer to the same thing across the three
module sets.
Add missing GStreamer modules to stable

Tarballs for gst-plugins-bad, gst-plugins-ugly, and gst-ffmpeg/gst-libav
are not distributed any more on GNOME sources, but they are on the
GStreamer FTP. This reinstates them in modulesets-stable, so that all
GStreamer modules and metamodules are now fully the same across all three
sets.
Stop skipping Flex.

The one in Xcode 5.0 and later blows up version detection.