Make these features solely dependent on the OS being built on.
This lets stack build on windows w/o XMPP, on OSX w/o DBUS,
and on Linux with everything.
Since stack is being used to build the OSX autouild now, I want xmpp
enabled.
This means stack can't be used to build git-annex on windows, unless the
user edits this file and disables xmpp. Unfortunate that stack is so
unconfigurable, compared with cabal..
Update install instructions.
Note that xmpp is disabled in the stack.yaml, because that needs C
libraries that would take extra work for the user to install.
xmpp is moving toward deprecation anyway.
