Reenable Vc for clang-7 toolchains.

Jö Fahlkerequested to merge
joe/ci:reenable-vc-for-clang-7 into master

Cmake will now refuse an incompatible vc version with clang 7+

Reverts !29 (merged)

See also core/dune-common!591 (merged)

Edited by Jö Fahlke

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