We now just list the names of such variables in .MAKEOVERRIDES.
When we come to export MAKEFLAGS we quote the value of each exported variable
using :Q, using: ${.MAKEOVERRIDES:O:u:@v@$v=${$v:Q}@}
The :O:u suppresses duplicate names.
Also modifed Parse_DoVar to re-export MAKEFLAGS whenever .MAKEOVERRIDES
is assigned to so .MAKEOVERRIDES+= PATH will export PATH=${PATH:Q}
to the environment, while .MAKEOVERRIDES= will disable export of VAR_CMD's.
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