minor, avoid parameter name `hz' due to AIX defining that in m_param.h
chore, remove ?= GNUism from yuck.am

This changeset assumes the M4 make variable has been set beforehand.
Merge branch 'bld/aix'

* bld/aix:
  chore, remove ?= GNUism from yuck.am
  minor, avoid parameter name `hz' due to AIX defining that in m_param.h