Only test for same cabal-version (but not ghc-ver)

The overly strict test was introduced in
4806aba73d57bcbac08d697046e387af77bd2506
but this breaks GHC bindists. Cabal 1.20 and older
also only checked the cabal version to match but
ignored the compiler versions.

Differing compiler versions per se are considered harmless.

Approved by @dcoutts