package/spice: don't enable GUI if dependencies not met

The spice GUI selects unconditionally selects cegui06, but cegui06 requires
threads and C++, so spice GUI must also depend on threads and C++.

Fixes:
    http://autobuild.buildroot.net/results/37f8ee90bb4348db97daef83493005c42a193b91/



Merry X-Mas! ;-)

[Peter: add SPICE_CLIENT dependency]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>