fix use of $(SED)

fix use of $(SED)

The alsactl Makefile used 'sed $(SED)' which is going to failed when SED
is actually set. Replaced that with '$(SED)', and a few 'sed' calls in
alsaconf Makefile as well. Added AC_PROG_SED to configure to have it
set.

Fixes: https://bugtrack.alsa-project.org/alsa-bug/view.php?id=5363
Fixes: http://bugs.gentoo.org/show_bug.cgi?id=366587
Signed-off-by: Michał Górny <mgorny@gentoo.org>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>