Fix premature setting of pkg-config extra_env

Also force native to 10.9 SDK for 10.9, since it has no /usr/include
Fix wrong -std flag passed to clang for pkg-config
Patch gtk-mac-integration  crash
Missed a chunk in the previous commit
Support multiple SDK directories

Via the SDKPATH, which is like every other PATH variable.
Also raises an EnvironmentError if it can’t find the designated
SDK.
Use “gcc” for the compiler name when Xcode is 4 or later.

This is a standard link to llvm-gcc-4.2 in Xcode 4 and to clang
in Xcode 5.

Also, raise an EnvironmentError if you try to build against the 10.4u
SDK with Xcode 4 or later: SDK 10.4u doesn’t work with gcc-4.2, and
only Xcode 3 and earlier support gcc-4.0.
Update gtksourceview3 to 3.10.1 for compatibility with gtk+-3.10
Xcode4+ linker workaround for pre-Lion SDKs
Upgrade gtk+-3.0 to gtk+-3.10.7

Needed to pick up gnome icons by default.
Add gnome theme

Gtk+-3.10 and later requires "gnome" themes for some of its standard
icons. This change adds those packages and make the core ones a
dependency of meta-gtk-osx-gtk3.
Fix misspelling
Update gtkmm3 and glibmm to match current gtk3 and glib versions
Fix compiler error in gtk+-3.10.7
Correct dependency, args for gnome-themes-standard

Required for gtk3, may well not have gtk2 installed
Update some modulesets versions to better match modulesets-stable
Fix typo
Fix another typo
More typos. sheesh.
Update libffi version

Fixes closure issue in gtkobject-integration.
Thanks to Vloker Braun
Force libxml2 to install its python module, and to do so in $PREFIX.