Comment (by Roland Haas):

@{1648a2da-81db-4efd-8c01-a5e594c71763} and I had a some offline discussion and iterated over the best regular expression to use. In he course of this we realised that $(...) cannot be used b/c there is already an option list that relies on $(…) being expanded as only a make variable to set C_MAKE_DEPENDS using CC which is only known to make but not in the environment (usually).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2312/expand-env-variables-in-rhs-of-option-list