Commit 7be41675 set -std=gnu99 for C code via QEMU_CFLAGS. Currently
we generate a "custom" QEMU_CXXFLAGS for VSS DLL C++ build by
filtering out some options from QEMU_CFLAGS and adding some others.
Since we don't filter out -std=gnu99 currently this breaks builds when
VSS support is enabled.

We could keep the existing approach, filter out -std=gnu99 from
QEMU_CFLAGS, and add -std=gnu++98, like configure currently does for
QEMU_CXXFLAGS, but as it turns out our resulting QEMU_CXXFLAGS would
be exactly what configure already generates, just with these filtered
out:

  -fstack-protector-all -fstack-protector-strong

and these added:

  -Wno-unknown-pragmas -Wno-delete-non-virtual-dtor

So fix the issue by re-using configure-generated QEMU_CXXFLAGS and
just handling these specific changes.

Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>