meson: pass more options directly as -D

If an option is not used anywhere by the configure script, it can be just
added to $meson_options even if it is not parsed by the automatically
generated bits in scripts/meson-buildoptions.sh.

The only slightly tricky case is $debug, where the

  if test "$fortify_source" = "yes" ; then
    QEMU_CFLAGS="-U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 $QEMU_CFLAGS"
    debug=no
  fi

assignment is dead; configure sets fortify_source=no whenever debug=yes.

Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>