spec: Explicitly don't build with c++

This is the historical intended behavior in the buildroot, but for
local builds, or with clang, qemu would detect a c++ compiler on the
host. So explicitly make the check fail by passing /bin/false

Signed-off-by: Cole Robinson <crobinso@redhat.com>

qemu.spec

@@ -1280,6 +1280,7 @@ mkdir -p %{static_builddir}
 
 run_configure() {
     ../configure  \
+        --cxx=/bin/false \
         --prefix="%{_prefix}" \
         --libdir="%{_libdir}" \
         --datadir="%{_datadir}" \