prefer having nasm/yasm but if its not there warn and fallback
to disabling assembler optional goodness

Change-Id: Ib31ad81717842f743c2910d575a9ebbc279a9c79
Reviewed-on: https://gerrit.libreoffice.org/35189Reviewed-by: Caolán McNamara <caolanm@redhat.com>
Tested-by: Caolán McNamara <caolanm@redhat.com>