Define PARENT
This allows makefiles to refer to the parent directory in an OS neutral way.
For example running a child makefile:
MkTables${SEP}maketables: MkTables${SEP}c${SEP}maketables
${CD} MkTables; ${MAKE} -f maketables_mk COMPONENT=maketables THROWBACK=${THROWBACK}; ${CD} ${PARENT}