build-sys: fix --enable-dbus

The incorrect shell code of AS_IF always get false value, so it always
build without dbus though build with "--enable-dbus" explicitely.
bus: break reference cycle between bus and messages

Previously (6ee4f99 bus: break reference cycle between bus and
messages) I committed the test code, but not the actual fix :)
build-sys: increase valgrind stack limit so all tests pass again