use GNUMake set-if-unset construct (?=) for build variables#1

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benkirk wants to merge 1 commit intoNCAR:mainfrom

benkirk:build_tweaks

Commits on Jan 3, 2025

use GNUMake set-if-unset construct (?=) for build variables, so we can use a common Make_defs and override select settings
benkirk
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