glib/gst-plugins-bad: Don't pull in dtls/datagram patches

We don't actually use those patches, and the code will likely change a lot
before it gets merged upstream.

See discussion on: https://bugzilla.gnome.org/show_bug.cgi?id=697907
gettext: Fix build on OSX

The preprocessor can't find headers if we don't pass -isysroot to it too.
Always include the sysroot in CC/CXX/OBJC/CPP already for iOS/OSX

Otherwise they might not find headers like stdio.h and others.
Add notes about it being a submodule.
Update README.md