build: make meson-buildoptions.sh stable

build: make meson-buildoptions.sh stable

The library directory can change depending on the multilib setup of the host.
It would be even better to detect it in configure with the same algorithm
that Meson uses, but the important thing to avoid confusing developers is
to have identical contents of scripts/meson-buildoptions.sh, independent
of the distro and architecture on which it was created.

So, for now just give a custom default value to libdir.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>