dbus: uses fork(), requires MMU

In order to solve
http://autobuild.buildroot.org/results/34f6843137efda20626af72714c110280ec577d7/build-end.log

,
this patch makes the D-Bus package as well as all the packages that
select the D-Bus package 'depends on BR2_USE_MMU'.

In addition, for the specific case of gvfs, the missing
BR2_TOOLCHAIN_HAS_THREADS dependency is added (threads are required by
D-Bus, so they are also required by gvfs which selects D-Bus).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>