makefile cleanups

makefile cleanups

0.7 requires dbus 1.1 or greater (for system bus activation), so make that
explicit, and remove compat code for D-Bus 0.6 and earlier.  Consolidate
the various glib pkgconfig checks into one, since most anything will require
gthread, glib, and gobject anyway.  Fixup the docs makefile to be more
automake-compatible and let 'make clean' actually work correctly when
docs are built.