Roman Bogorodskiy reported that attempts to use 'make syntax-check'
for libvirt on a BSD host was failing, due to libvirt's use of
GNU sed constructs in cfg.mk. While libvirt could be patched to
use only portable sed constructs, it is also worth making sed
replaceable so that any maintainer can use a consistent sed program
rather than being hardcoded to the first 'sed' on $PATH.
* modules/maintainer-makefile (configure.ac): Check for sane sed.
* top/maint.mk: Change sed to $(SED).